Gay rights campaigners will lead a major parade through Belfast city centre later, demanding the introduction of same-sex marriage in Northern Ireland.
This afternoon's demonstration will conclude with a rally in front of Belfast City Hall.
Trade union members, officials and activists in the Republic of Ireland are being urged to take the 'Love Train' from Dublin to Belfast to show support.
The train leaves Dublin's Connolly Station at 11.20am with marchers gathering at Belfast's Writer's Square from 2.30pm for the 'love equality rally'.
The Irish Congress of Trade Union's Equality Officer, David Joyce, says its a gesture of solidarity with the campaign in the North.
The ban on same-sex marriage is one of a series of sticking points holding up the formation of a new powersharing government at Stormont.
Northern Ireland is the only part of the UK and Ireland where same-sex marriage remains outlawed.
The DUP opposition to changing the law has attracted increased scrutiny across the UK since the party became the UK Government's kingmaker at Westminster.
Ahead of the event, a range of celebrities including Liam Neeson, Stephen Fry and Graham Norton voiced their support for the campaign.
Scottish First Minister Nicola Sturgeon also joined the leaders of all the main parties at Holyrood to call for a law change across the Irish Sea.
Amnesty International is one of the groups organising the parade, which will be headed by Belfast Lord Mayor Nuala McAllister.
NEWS: Thousands expected on Belfast march for #equalmarriage. Lord Mayor to help lead march to rally at City Hall. https://t.co/0i0eDloaFG pic.twitter.com/zPh0cLtZwn— Amnesty Int'l NI (@AmnestyNI) June 30, 2017
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.
'Demand Change' is the theme for Belfast Pride 2017. Celebrate your life. Demand Equality. Demand Change. See more: https://t.co/jawPbFjMYk pic.twitter.com/m1H12aodi6— Belfast Pride (@belfastpride) June 28, 2017
That will only be tested once, and if a devolved Assembly can be re-established out of the current political crisis in Belfast.
If the North's politicians 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 region's marriage laws would be handed to the London government.