A Brisbane rally opposing the plebiscite on same-sex marriage has drawn 2000 supporters.

Thousands of marriage equality supporters opposed to the coalition government's plebiscite have marched on Brisbane.

The estimated 3000-strong crowd at Queens Park on Saturday was hailed by organisers as the city's biggest same-sex marriage rally.

Speakers at the event criticised the non-binding plebiscite, which the Liberals have promised to hold if they win the election, as a "slap in the face".

Openly-gay Labor candidate for Brisbane Pat O'Neill said July 2 was the opportunity to give the LGBT community a "voice at the ballot box".

"Put Malcolm Turnbull last, that's where he's putting us," Mr O'Neill said.

Greens senator Larissa Waters told the crowd the plebiscite was just a $160 million opinion poll to "take the temperature of the Australian community".

"It's not going to be binding on their members - what a joke," Ms Waters said.

"That doesn't sound like democracy to me and it certainly doesn't sound like leadership."

Independent senator Glenn Lazarus, who reversed his support for a plebiscite after chairing a committee report into it, said same-sex marriage was a human rights issue.

"It does effect thousands if not millions of people across this country," Mr Lazarus said.

"We need to get it sorted."

Prime Minister Malcolm Turnbull this week stated he expected the overwhelming majority of federal politicians to back same-sex marriage legislation in parliament if a plebiscite on the issue was successful.

Rallies opposing the plebiscite were also held in Sydney and Melbourne.