A demonstration at 3pm at Belfast City Hall was organised by The Rainbow Project, Northern Ireland's main LGBTI organisation and was addressed by representatives from Amnesty International and trade unions.
Alliance MP Naomi Long, Sinn Féin MLA Gerry Kelly and SDLP MLA Alex Attwood also made speeches at the protest.
Parallel demonstrations were also staged in Londonderry and Newry.
They were in protest against a Private Members' Bill proposed by DUP MLA Paul Givan, which, if made law, would create a legal clause allowing the refusal of goods and services on the grounds of strongly held religious beliefs.
The action was prompted by a case taken by the Equality Commission against a Christian family run bakery which refused to bake a cake featuring a gay rights slogan, with the DUP backing the decision by Ashers.
John O'Doherty, director of The Rainbow Project commented ahead of the rallies: "We believe this conscience clause will create a licence to discriminate for those who oppose the LGBT people here in Northern Ireland.
"We don't believe we should be treated as second-class citizens. We want to ensure equality for our entire community and we're looking forward to many people across Belfast, Derry and Newry standing in solidarity with us today in opposition to this draconian legislation."
Amnesty International has argued the proposed law would allow anti-gay discrimination.
Northern Ireland Programme Director of Amnesty International, Patrick Corrigan, said: "What is proposed is not a conscience clause, it is a discrimination clause.
"This change to the law is not welcome and it is not needed. The law already strikes a fair balance between the human right to freedom of religion and the human right not to suffer discrimination."
Mr Corrigan added: "Northern Ireland's First Minister should concentrate on eradicating inequalities already faced by members of the LGBTI community here, rather than lending his support to a discriminatory new law.
"He could start by publishing Stormont's long overdue sexual orientation strategy, which could help tackle homophobia in Northern Ireland society."
TV personality Stephen Fry is one of almost 19,000 people who have signed a petition against the Bill.