Scotland's political leaders have issued a joint call for the introduction of same-sex marriage in Northern Ireland.

First Minister and SNP leader Nicola Sturgeon, Scottish Conservative leader Ruth Davidson, Labour's Kezia Dugdale, Patrick Harvie from the Green Party and Liberal Democrat Willie Rennie all voiced support for a law change across the Irish Sea.

Their intervention comes ahead of a major demonstration by gay rights activists in Belfast tomorrow.

The issue of same-sex marriage is one of a series of sticking points holding up the formation of a new power sharing government at Stormont.

The DUP's opposition to changing the law has attracted increased scrutiny in Britain since the party became the Tory government's kingmaker at Westminster.

Ms Sturgeon said she was proud that gay marriage had been introduced in Scotland.

"The debate over equal marriage in Scotland did more than just simply allow people to marry, it also helped to challenge negative attitudes that still exist today in our society towards LGBTI people and show, quite simply, that same-sex couples are just as valued as opposite sex couples," she said.

"I offer my support to those continuing the campaign for marriage equality in Northern Ireland."

Scottish Tory leader Ms Davidson said: "I'm proud to support those campaigning for marriage equality and I'm optimistic that this is a battle that can be won.

"Equal marriage isn't about one religion or country or community. It's much simpler than that. At its heart, equal marriage is about the people of Northern Ireland being afforded the same rights as everybody else.

"So as thousands march in Belfast this Saturday, I have this message for them. Change is coming. And it's a change for the better."

Scottish Labour leader Ms Dugdale echoed the comments.

"Scotland, England, Wales and the Republic of Ireland have all now backed equal marriage, and it has been liberating and encouraging to see such progress make the news and headlines in places around the world where marriage equality is sadly still so far away," she said.

"I'm backing the campaign for marriage equality in Northern Ireland.

"I know there will be opposing views on this and the debate must be conducted courteously, but I firmly believe it's time for LGBT people across the whole of the UK to enjoy the right to marry who they want."

Co-convener of the Scottish Green Party Mr Harvie said: "We must not settle for the current treatment of same-sex couples in Northern Ireland, or just accept that the prejudice of the DUP will never be overcome.

"Most people in Northern Ireland support marriage equality, a majority of the Assembly supports marriage equality, and we will campaign relentlessly with our friends in Northern Ireland until they achieve that goal."

Scottish Liberal Democrat leader Mr Rennie said equality in marriage for everyone was a "mark of a society".

"The world has not collapsed since we introduced it in Scotland," he said.

"In fact, it has brought joy to many lives. I would urge the political representatives of Northern Ireland to embrace it too."

Amnesty International, which is one of the leading members of the Love Equality coalition organising the march, released the statements from Scotland's political leaders.

The DUP has used a controversial Stormont voting mechanism - the petition of concern - to prevent a law change, despite a majority of MLAs supporting the move at the last vote.

The party rejects any suggestion it is homophobic, insisting it is instead protecting the "traditional" definition of marriage.

Following March's snap Assembly election, the DUP no longer has the electoral strength to deploy a petition of concern in its own right, though it could still potentially combine with other socially conservative MLAs to do so.

That will only be tested once, and if a devolved Assembly can be re-established out of the current political crisis in Belfast.

If politicians in Belfast fail to establish a new executive, direct rule from Westminster could be re-imposed.

If that were to happen, the responsibility for legislating on the north's marriage laws would be handed to the London government.