Thousands of anti-Israel and anti-war protesters clashed with police outside a defence expo in downtown Melbourne on Wednesday, leading to dozens of arrests.

Victoria state police said the demonstrators pelted officers with rocks, acid and suspected human feces as violence broke out in Australia's second-largest city.

In what the Australian media said was the largest show of force in the city since 2000, when Melbourne hosted the World Economic Forum, more than 1,000 police officers were involved in the operation.

They deployed sponge grenades, flash-bang devices and irritant sprays to control the rioters. Law enforcement said that 24 officers required medical attention and 39 people were arrested. The offences included assaulting, obstructing or hindering police, arson and blocking roads.

“Victoria police is appalled at the behavior of some of the protesters in attendance. If you wish to come and protest, do so peacefully. We will not tolerate criminal behavior," the Victoria state police spokesperson said.

“Some police have been spat at by protesters, whilst other officers have been sprayed with a liquid irritant, some of which has been identified as acid," the spokesperson continued.

The protests were organised by a collection of groups calling itself Disrupt Land Forces, including the Australia Palestine Advocacy Network. Videos and photos showed many rioters waving PLO flags and chanting anti-Israel and pro-Palestinian slogans through loudspeakers.

“From the river to the sea, Palestine will be free," a chant calling for the elimination of Israel, was heard at the protest.

They were protesting against the biennial Land Forces International Land Defence Exposition being held at the Melbourne Convention and Exhibition Centre.

Around 1,000 exhibiting organisations from 31 countries, including Israel, were expected at the expo, which organizers said was the largest in Australia.

The event wraps up on Friday, with the protests continuing through Saturday.