Braddon's Lonsdale Street is set to close as Canberrans react to results from the government's same-sex marriage postal vote on Wednesday.
Campaign group Australian Marriage Equality received approval from the ACT government to close part of the road near Hopscotch bar from 6pm, hours after the national survey results are released.
There is growing optimism among campaigners for same-sex marriage with similar events planned around the country, including on Sydney's Oxford Street.
Australian Marriage Equality ACT organiser Jacob White said hundreds of people were expected to attend the Canberra street party.
"If there's a yes announcement, which we're hoping for, I think a lot of people will get excited and look for somewhere to be with some friendly and familiar people," he said.
The street party will kick off at 5pm, when Hopscotch will open its doors and people will be invited to chalk the street rainbow.
A 2-metre high balloon rainbow "yes" will cross the road. Several Lonsdale Street vendors will cater to party-goers with interstate DJs to provide the music.
WA Liberal senator Dean Smith, Labor's Penny Wong and ACT Chief Minister Andrew Barr will speak at the event.
"There are many reasons to be proud of our city and we are particularly proud of how Canberrans responded to the marriage equality postal survey," Mr Barr said.
"Fairness and equality are at the heart of Australian society. This event celebrates all Australians having the same opportunity for love and happiness."
Counsellors will be on hand to provide support should the "no" campaign prevail, though Mr White said the organisation was "not considering that option to a great extent at this point".
"The title of the event is 'no matter what we will party" ... at the end of the day, this campaign has had in Canberra alone hundreds of people involved in it and they've been through a lot in the past few months and we want them to get together and have a fun time regardless," he said.
On Wednesday, Canberra "yes" supporters will gather in Haig Park from 9am, watching the 10am announcement from the Australian Bureau of Statistics live.