Supporters young and old with rainbow flags and handmade signs flocked to the steps of the State Library on Saturday as part of a rally organised by the Equal Love group.

Equal Love campaigner Ali Hogg said around 20,000 people had come to Melbourne to show their support for same sex marriage.

"It's an amazing turnout…we are so elated," Ms Hogg said.

She said it was the second-biggest rally for marriage equality in the country and was a sign of the huge amount of support in the public for the cause.

"After today we are very confident that we can get a majority voting 'yes' for this survey," she said.

Naomi Goldwater said it was "phenomenal" to see so many people spend their Saturday at the rally for equality.

"It was important for me to go to show the LGBTIQ community that they have massive community support, especially young people being exposed to awful things at the moment," she said.

The 'yes' vote supporter said the postal vote was a "total abrogation" of the parliament's job to make laws.

Another attendee Gemma Cafarella said it was important to send a strong message to young LGBTIQ kids.

"They're valued and loved and that there's nothing wrong with them," she said.

Federal Labor frontbencher Mark Dreyfus said it was just a matter of time until marriage equality is achieved in Australia.

"It's going to happen. It's either going to happen through this process, or whatever process by a private member's bill," he told the crowd.