Update 1.05pm: Donald Tusk, President of the European Council, said: "Sad to learn of sudden passing of Martin McGuinness. His contribution to peace in Northern Ireland will endure."
The European Council president tweeted: "Sad to learn of sudden passing of Martin McGuinness. His contribution to peace in Northern Ireland will endure.
"Ar dheis De go raibh a anam," he added.
Sad to learn of sudden passing of Martin McGuinness. His contribution to peace in Northern Ireland will endure. Ar dheis Dé go raibh a anam.— Donald Tusk (@eucopresident) March 21, 2017
Britain's Queen Elizabeth is sending a private message to the family of Martin McGuinness following his death.
The monarch and the former IRA commander met and shook hands at a charity event in Belfast in June 2012, an event which would once have been unimaginable.
Buckingham Palace indicated that the Queen would be contacting his wife Bernie following his death on Tuesday.
Update 12.15pm: The former leader of the SDLP in the North, John Hume, and his wife Pat have expressed their sympathies after hearing of the death of Martin McGuinness.
Mr Hume, a fellow Derry man and a co-recipient of the 1998 Nobel Peace Prize for his role in the North's peace process, said: "Martin McGuinness left an indelible print on politics in Ireland. Our starting points on the approach to finding a solution to the divisions on this island were very different but there can be no doubt Martin was deeply committed to the agreed Ireland of the Good Friday Agreement and its power sharing institutions.
"He played a very important role in our peace process and his courageous, generous and gracious gestures as deputy First Minister were offered in a spirit of reconciliation and peace building.
"Our deepest condolences go to his wife Bernie, their children Fiachra, Emmet, Grainne, Fionnuala and the wider family circle."
Sinn Féin Deputy Leader Mary Lou McDonald has also paid tribute to Mr McGuinness.
Speaking at Leinster House today Ms McDonald said: “Today we have not only lost a great Irish leader and patriot, we have also lost our friend.
“Martin was an inspirational Irish Republican and a tireless worker for peace, reconciliation and Irish Unity. It was my privilege to call him my friend. He will never be forgotten.
“His legacy is the peaceful future he helped to secure for our island and the advancement of the Irish Unity movement which is gathering pace at this time.
“Our thoughts and condolences are with Martin’s clan, his wife Bernie, his children Fiachra, Emmet, Fionnuala and Grainne, his granchildren and extended family.
“I measc laochra na nGael go raibh a anam dílis.”
Update 11.45am: Archbishop Eamon Martin has spoken of his sadness at the death of Martin McGuinness.
It comes after the former deputy first monister at Stormont died this morning after a short illness.
Archbishop Martin said: "Like many people I was shocked before Christmas to hear about the serious illness of Martin McGuinness, and, despite our hopes and prayers for his recovery, today I am saddened to learn that he has died. My first thoughts are with his dear wife Bernie, his children, grandchildren, brothers and sister, and all his many friends and loved ones.
"As a leader he was courageous and took risks in order to bring others with him, convincing them that goals could be achieved by politics and persuasion. He channelled his many gifts into creating and sustaining the peace process of which he was one of the key architects. I have no doubt that Martin's faith and relationship with God guided him along this journey. He was a man of prayer and I am personally grateful for his good wishes and encouragement to me, as a fellow Derry man, in my own vocation."
The Primate of All Ireland went on to talk about the Sinn Féin politician's contribution to the Northern Ireland Peace Process.
Archbishop Martin said: "The story of conflict in Ireland has brought much pain and trauma and I thank God that in recent years we have preferred peace to the horror of violence and war. People like Martin McGuinness have made an immense contribution to sustaining peace by reaching out a hand of friendship and reconciliation and being prepared to model alternatives to dispute and division.
"Martin's personal warmth and open, friendly personality was able to melt away suspicion and help build trust with those coming from very different perspectives. Being grounded in love for his family, community and native city of Derry, he understood the importance of a peaceful, just and prosperous future for all.
"Martin was ambitious for peace. He knew that peace was worth striving for and was within reach in his life time."
The Archbishop of Armagh concluded by urging the political parties in the North to continue efforts to resolve the current political impasse there.
He said: "A fitting legacy for Martin would be a redoubling of efforts on all our parts to find solutions to our current problems and continue along the journey to a shared future.
"May he rest in peace. Ar dheis Dé go raibh a anam dilis."
The Bishop of Derry, Bishop Donal McKeown, said McGuinness' death will affect many people in different ways.
He said: "Martin McGuinness played a uniquely significant role in the life of this society for almost fifty years. Firstly my sympathies go to his wife Bernie and family. They have lost someone, who remained so loyal to and protective of them throughout his life.
"Secondly, for those who know him well, there will be a huge sense of personal loss. Martin’s role in moving Northern Ireland from conflict to politics has been immense.
"Others will have more painful memories of how their lives were affected. And I encourage everyone, when they reflect on Martin McGuinness, to be sensitive to how his death will be felt across this community.
"I pray that, as a society, we will be able to reflect on the life and death of Martin McGuinness in ways that help to move us all forward. Ar dheis Dé go raibh a anam dilis."
Update 10am: Fianna Fáil Leader Micheál Martin said Martin McGuinness "worked to build bridges between the different traditions and communities on the island.”
Mr Martin was speaking about the news that the politician died in the early hours of this morning.
"More than most Martin Mc Guinness embraced the peace process with a generosity of spirt that won people over.
"By his actions and words over the last 25 years, he demonstrated a keen understanding of what the peace process was all about.”
Update 8.30am:Former British Prime Minister Tony Blair acknowledged Martin McGuinness’s role in the Good Friday agreement after the Sinn Féin politician died this morning.
“I grew up watching and hearing about the Martin McGuinness who was a leading member of the IRA engaged in armed struggle. I came to know the Martin McGuinness who set aside that armed struggle in favour of making peace. There will be some who cannot forget the bitter legacy of the war. And for those who lost loved ones in it that is completely understandable.
“But for those of us able finally to bring about the Northern Ireland peace agreement, we know we could never have done it without Martin's leadership, courage and quiet insistence that the past should not define the future.
“I will remember him therefore with immense gratitude for the part he played in the peace process and with genuine affection for the man I came to know and admire for his contribution to peace."
Current UK Prime Minister Theresa May said: "While I can never condone the path he took in the earlier part of his life, Martin McGuinness ultimately played a defining role in leading the Republican movement away from violence.”
Labour Leader and spokesperson on Northern Ireland, Brendan Howlin said: "The role Martin played in bringing peace and reconciliation to our island will never be forgotten.
"Over the coming days as we remember Martin's life we will appreciate the enormous road we have travelled on this island since the darkest days of the Troubles.”
Update 7.50am: Taoiseach Enda Kenny has paid tribute to Martin McGuinness, who died in the early hours of this morning, aged 66.
“His passing represents a significant loss, not only to politics in Northern Ireland but to the wider political landscape on this island and beyond.
“Martin will always be remembered for the remarkable political journey that he undertook in his lifetime. Not only did Martin come to believe that peace must prevail, he committed himself to working tirelessly to that end."
"Martin was one of the chief architects of the Good Friday Agreement and he worked resolutely in the years that followed it in pursuit of its full implementation.
“His commitment to securing enduring peace and prosperity for all of the people of Northern Ireland was unwavering throughout this time. He strove to make Northern Ireland a better place for everyone, regardless of background or tradition."
Democratic Unionist leader and former Stormont first minister Arlene Foster, with whom Mr McGuinness recently had a fractured relationship, said she offered "sincere condolences".
Mr McGuinness stood down in January in protest at the DUP's handling of the 'cash for ash' energy scandal, triggering a snap election.
"Today's news will come as a shock to many people," said Ms Foster.
"First and foremost, Martin McGuinness was a much loved husband, father and grandfather.
"My thoughts and prayers are with his wife and the family circle at this very painful time of grief and loss."
She said history would record differing views on Mr McGuinness but he had played a pivotal role in bringing the republican movement toward peace.
Mr McGuinness's successor as the party's leader at Stormont, Michelle O'Neill, paid tribute.
She tweeted: "My heart is broke this morning. We have lost a legend, a giant of a man. I'm very proud to say he was my friend and mentor x."
My heart is broke this morning. We have lost a legend, a giant of a man. I'm very proud to say he was my friend and mentor x— michelle oneill (@moneillsf) March 21, 2017
Minister for Foreign Affairs and Trade, Charlie Flanagan, also paid tribute.
"It was with great sorrow I learnt of Martin's passing. My deepest sympathies are with his wife Bernie and his family at this very sad time.
“Martin and I come from very different political traditions. However, in his embrace of the politics of peace, he made an immense personal contribution to building and consolidating peace on this island.
“His own personal journey from militant republicanism to deputy First Minister in a power-sharing administration with unionism helped to map the road to the Good Friday Agreement and its vision of partnership and reconciliation.”
Former Taoiseach Bertie Ahern said it was a brave move for Mr McGuinness to leave the IRA.
“What isn't argued is that he made a transformation and moved into the political force, then became an advocate for peace and a huge critic of those who wanted to be involved in dissident republicanism; spoke out stronger than anybody against that.”
Earlier: President Michael D. Higgins has expressed his sorrow at news of the death of Martin McGuinness.
"It was with great sadness that I have heard of the passing of Martin McGuinness, and on behalf of Sabina and myself, may I express our deepest sympathy to his wife Bernadette and to his family,” President Higgins said.
“The world of politics and the people across this island will miss the leadership he gave, shown most clearly during the difficult times of the peace process, and his commitment to the values of genuine democracy that he demonstrated in the development of the institutions in Northern Ireland.
“As President of Ireland, I wish to pay tribute to his immense contribution to the advancement of peace and reconciliation in Northern Ireland - a contribution which has rightly been recognised across all shades of opinion.”
President Higgins also paid tribute to Mr McGuinness on a personal level and referenced their shared love of sport.
“Sabina and I have appreciated both Martin McGuinness' warmth and his unfailing courtesy.
“Our paths have crossed many times in recent years at official events, including most recently at the GPO for the 1916 commemorations, as well as in our celebrations of 'the beautiful game', at Glentoran and in France at the European Championships.
“In addition to his services in public life, as an inclusive believer in community in all its forms he will also be remembered for his warm support for Derry GAA and Derry FC, having been an outstanding, championship winning Football player in Derry's U21 and senior teams."