MLAs will later debate at Stormont the thorny issue of same sex marriages.
Sinn Féin has tabled a motion calling for same sex marriage to be legalised in Northern Ireland.
It followed a decision by MPs in England and Wales to make it legal there.
However, the DUP has lodged a petition of concern in a bid to block the motion. It would need cross-community support to pass.
Campaigners in favour of same-sex marriage will hold a demonstration outside Parliament Buildings ahead of Monday’s debate, which comes after a similar motion failed at the Assembly last year.
Amnesty International has warned there could be a legal challenge if NI is left as the only part of the UK without marriage rights for same-sex couples.
Patrick Corrigan said: “States may not discriminate with regards to the right to marry and found a family, on the basis of sexual orientation or gender identity.
“That obligation is clear in international law. This means that marriage should be available to same-sex couples in NI just as it appears it soon will be in other parts of the UK.
“Should politicians fail to act, there could be a straightforward legal challenge on the basis of inferior treatment of same-sex couples in Northern Ireland with regards to the right to marry and found a family.”
The first same sex civil partnership in the UK took place in Belfast in December 2005, but same sex marriage is still illegal.
The Presbyterian and Catholic churches wrote to MLAs last week urging them to reject the motion and resist any change to the law.
Representatives from the Equal Marriage NI campaign say they will be protesting in favour of the motion on Monday.
John O’Doherty said: “Members of the LGBT community and their allies will be present at Parliament Buildings to show support to the parties and individual members of the Assembly who will put themselves on the right side of history and vote in favour of this motion.”
DUP Chief Whip Peter Weir said the debate “will only result in further embarrassment for those parties and individuals who avoid telling the electorate where they stand”.
He added: “It has been made very clear that same sex marriage will not be introduced in NI and the DUP is tabling a Petition of Concern to ensure that this motion will not be carried.”
However, former Sinn Féin Education Minister Catríona Ruane argued the issue of same sex marriage was widely supported at the constitutional convention, which was comprised of political representatives from across the island and members of the public.S
Said the South Down MLA: “An overwhelming majority of those 99 people, voted for equal marriage, 81% voted that the state shall protect and provide for same sex marriage, they also voted for children’s rights to be protected.”