A leader of Northern Ireland's civil rights movement in the 1960s, says Brexit and a hard border will damage human rights.
Michael Farrell was speaking ahead of today's 50th anniversary of the infamous October 5 march in Derry in 1968, when he and a number of others were beaten by Royal Ulster Constabulary (RUC) members during a demonstration calling for housing rights in the city.
Fifty years on, the civil rights movement is seen by the nationalist community as a key force in gaining equality in the North, whereas unionist critics have labelled it as a Trojan horse for furthering the cause Irish reunification.
"After thirty years of terrible violence, we eventually got a settlement, based on the same things we were looking for at the beginning, back in the 60s - a fair and just society," Mr Farrell said.
"Nondiscrimination, human rights, equality - these are an essential part of the Good Friday Agreement, I'm very worried by Brexit as it does threaten to undermine that.
"There is a real danger now that if a border is reinstated with checks and so on, people will believe things have gone back 30 or 40 years and the agreement has not achieved what they thought," he said.
Mr Farrell become involved in the Northern Ireland civil rights movement in the late 1960s, while at Queens University in Belfast, and was a founding member of the university-based People's Democracy group.
After moving to Dublin and becoming a solicitor, Mr Farrell was co-chairman of the Irish Council for Civil Liberties, a member of the Irish Human Rights Commission and the European Commission against Racism and Intolerance, and was appointed to the Irish Council of State by President Michael D Higgins.
"Within the Good Friday Agreement there are a range of legal protections for people's human rights, as the European Convention of Human Rights has been brought into domestic law, meaning Stormont's actions can be struck down by the European Court," Mr Farrell added.
"The European Union law brought in a lot of the equality provisions in Northern Ireland, not just between unionist and nationalists, but equality within race or gender too.
"The Conservative Government want to repeal the European Human Rights Act. If Brexit goes through, the Charter of Human Rights from the European Union will cease from the day that Brexit is completed.
"Those laws underpin the Good Friday Agreement, and if you pull them out, and create a hard border, it will create huge disillusionment in Northern Ireland.
Civil Rights March in Derry #OnThisDay 50 years ago - often cited as the start of what became known as “The Troubles” in Northern Ireland.October 5, 2018
"I think it will give rise to a lot of trouble, demonstrations and so on, that could easily lead to some level of violence.
"I think that it is very alarming to anyone who believes in rule of law, who believes this is the way to run a society, especially a divided society, to say that the Good Friday Agreement could be changed or revised. (It) is very worrying."
A number of events are being held in Derry over the weekend as part of a special programme to commemorate the 50th Anniversary of Duke Street civil rights march, including an address by Irish President Michael D Higgins.