Martin McGuinness urged voters to "put Ireland first" as he launched Sinn Féin's campaign backing a Remain vote in the EU referendum.
The veteran republican said the European Union was far from perfect, but insisted the best way to reform it was from within.
Sinn Féin, the Ulster Unionists, SDLP and Alliance Party all support a Remain vote, while the Democratic Unionists are campaigning for Brexit.
At the launch event in a Belfast hotel, Mr McGuinness said: "Sinn Féin will be campaigning vigorously in the coming days and weeks to encourage people to vote to put Ireland first and vote Remain in the EU.
"The European Union is far from perfect but the only way to address that and change it is from within. Our policy towards the European Union remains one of critical engagement.
"Brexit would be bad for Ireland, bad for business and trade, bad for our farmers and bad for human rights and workers' rights.
"Many aspects of our society from community groups to business and education to agriculture have been able to grow and expand as a result of the support they have received from the European Union.
"We cannot allow the narrow interests of a section of the Tory party, which was not elected by the people of the North, to take us out of Europe and set our political agenda."
Sinn Féin MEP for Northern Ireland, Martina Anderson, also spoke at the event.
"This is a hugely important vote, which will have implications not just for the north but for the entire island of Ireland," she said.
"Brexit would be disastrous for Ireland north and south. We in the north, would be shackled to a Britain with a hostile, hawkish Tory government, wedded to the failed policies of austerity - a peripheral region of a peripheral state on the periphery of Europe.
"We critically engage with the EU, supporting what is right and good for Ireland while challenge shortcomings wherever we find them. And in doing so we want to build a better Europe.
"We in Sinn Féin have absolutely no confidence whatsoever that the British government has any interest in replacing funds and support lost to the north in the event of a Brexit.
"In order to build a better European Union, with a strong all-Ireland team of Sinn Féin MEPs, we need to vote Remain."