Northern Ireland is the only part of the country in which marriage equality does not exist.
Thousands of people are taking part in a demonstration in Belfast city centre to urge the government to rectify this.
The march set off off from Writers' Square and will culminate in a rally at Belfast City Hall.
— Amnesty Int'l NI (@AmnestyNI) July 1, 2017
The ban is a major issue for NI.
The Northern Ireland Assembly has already voted five times on whether to introduce same-sex marriage.
Celebrities such as Liam Neeson, Gary Lightbody, Graham Norton and Neil Hannon have given their support to the campaign.
Hi everyone! Today is the day! If you are volunteering or want to help come to the Belfast LGB&T centre for 1pm. Thank you! pic.twitter.com/6PJShVNwAJ
— Love Equality NI (@Love_EqualityNI) July 1, 2017
'My own mother was from Northern Ireland, so of course I have a huge affection for the place and its people,' said Graham Norton.
'I know it is hugely frustrating for gay people there that it is the last part of these islands still without marriage equality.'
'Especially when there is such overwhelming support for it among the public.'