Australian anti-war protesters on Thursday demonstrated for a second day against a military, defense expo being held in Melbourne.

Activist groups – including Extinction Rebellion and Students for Palestine – have united for the protest, which they said was triggered by Canberra's stance on Israel's genocidal war on Gaza.

The protests continued despite at least 33 people being arrested a day earlier when they clashed with police against the three-day convention, showcasing weapons and technology from around the world.

"We're protesting to stand up for all those who have been killed by the type of weapons on display at the convention," spokeswoman Jasmine Duff told The Age newspaper.

Local police, fearing crowds of as many as 25,000 protesters, had diverted resources from regional areas in anticipation – making it the force's biggest operation in 20 years.

But police said only 1,200 turned up Wednesday, many of whom yelled at expo attendees before clashing with police.

"Victoria Police is appalled at the behaviour of some of the protestors in attendance at the Land Forces protest in Melbourne's CBD this morning," the state police force said in a statement.

Police said they had been "pelted" with bottles, rocks and horse manure while trying to protect expo attendees, some of whom were also assaulted by protesters.

Protesters also sprayed officers with "a liquid irritant, some of which has been identified as acid," police claimed.

Demonstrators set bins and wooden pallets alight and formed a make-shift barricade, according to the media.

"Whilst we respect the right for individuals to protest lawfully, we will not tolerate the behavior of those who break the law," police said.

Prime Minister Anthony Albanese said people had a right to protest, but he disagreed with violence.

"People have a right to protest peacefully, but you don't say you're opposed to defence equipment by throwing things at police.

"They've got a job to do and our police officers should be respected at all times," he said.