Media playback is unsupported on your device Media caption John O'Doherty, chair of Equal Marriage NI, said they would keep trying until they succeed

Separate protests have taken place as MLAs at Stormont are set to debate a Sinn Fein call for same-sex marriage to be legalised in Northern Ireland.

Pro-gay marriage protesters gathered at Stormont and in Londonderry.

The DUP has already indicated an intention to use a petition of concern, which is effectively a veto.

However, one of the protesters, John O'Doherty, chair of the equal marriage NI campaign, said they would keep trying until they succeeded.

A similar motion was defeated in the assembly last October.

Speaking at the protest at Guildhall Square in Derry, Eamon McCann said: "I think it's tremendously important that we highlight the continued oppression of gay and lesbian and bi- and transgendered people, particularly in a society where so much was made for so many years about the need for equality.

Meanwhile, Amnesty International has warned of a likely legal challenge if Northern Ireland is left as the only part of the UK without marriage rights for same-sex couples.

The DUP's petition of concern would ensure that the motion would need to command a cross-community majority to succeed.

'Clear obligation'

Image caption Several people attended a protest at Stormont shortly before the gay marriage debate began

Patrick Corrigan, Amnesty International's Northern Ireland programme director, 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 Northern Ireland 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."

Mr Corrigan believes such a legal challenge seems likely and would appear to have strong grounds for success.

DUP Chief Whip Peter Weir confirmed the party has tabled a Petition of Concern on the motion, which has been tabled by Sinn Fein.

Mr Weir said: "It is only a few months since the assembly last debated this issue and views were clearly expressed at that time.

"It has been made very clear that same-sex marriage will not be introduced in Northern Ireland and the DUP is tabling a Petition of Concern to ensure that this motion will not be carried."