"This is one of over 120 events happening simultaneously around the world right now," vigil co-organiser and Jodie Harrison told AAP.
Between 30 and 50 people had gathered at the British consulate in Melbourne on Tuesday evening.
"The final court hearing in the UK, which thousands of our supporters have arrived from around the world for, is the last chance for Julian to avoid the US extradition," Ms Harrison said.
Assange, 52, is wanted in the US for an alleged conspiracy to obtain and disclose national defence information after Wikileaks published troves of US military records and diplomatic cables.
Assange has been battling extradition in UK courts for 13 years and spent seven holed up in the Ecuadorian embassy in London before being dragged out and charged with bail breaches.
He has been held in the maximum-security Belmarsh prison since.
Last week the Australia's federal House of Representatives passed a motion which urged the US and UK governments to allow Assange to return to Australia.
Addressing the vigil in Melbourne, PEN international representative Con Pakavakis said it was the British justice system that was on trial.
"The extradition treaty between the UK and the US clearly states there should be no extradition in political cases," Mr Pakavakis said.
"There's nothing normal about this case."
The UK hearing is expected to take two days and will begin at 9.30pm AEDT.
"If the US can get Julian Assange nobody is safe," Ms Harrison told AAP.
"If they succeed in this extraterritorial reach, in going after an Australian journalist and publisher who wasn't even in their country when this happened, then what is stopping them or any other country going after any journalist?"
8°C