Victoria Police say they are appalled at the behaviour of protesters who lobbed missiles and acid at officers in violent scenes in the Melbourne CBD.

Loud bangs rang out across the CBD on Wednesday as police pelted antiwar marchers with tear gas and pepper spray in clashes that began outside the Melbourne Convention and Exhibition Centre and spread to elsewhere in the city.

Victoria Police Chief Commissioner Shane Patten said later on Wednesday the thousands protesting the Land Forces defence exhibition were far from peaceful.

“The police officers on the ground were subjected to a whole range of vitriol to abuse; they were assaulted, they were pushed, they were shoved, they had bottles thrown at them,” he said.

“They had balloons thrown at them that had different liquids in them. We’ve had analysis that shows some type of low-level acid that caused irritants to the police was used.”

Patten said some officers had complained of faeces being thrown at them. That was still being confirmed, although horse dung was lobbed.

“This is the type of disgusting behaviour that we saw today from a group who were intent on confronting us at this stage,” he said.

“Fortunately, we don’t believe any of our officers have significant injuries – although it’s not insignificant when you’ve been pelted in the eye and you’re treated for something in the eye, you’ve been hit in the head.”

He said there had been 39 arrests for charges including assaulting, hindering and obstructing police, general assault, arson and blocking roads.

Pro-Palestinian Protests in Melbourne pic.twitter.com/2mIO4RZ1fU

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Earlier, Victorian Premier Jacinta Allan said it was “a potentially dangerous and dynamic situation”. She estimated about 3000 people were involved in the protests, down from the 25,000 that rally organisers had predicted.

Allan said police were doing their job to support community safety.

“They deserve to be treated absolutely with respect by people who are attending this protest,” she said.

“I say to anyone going down to threaten police, to threaten community safety, [you] will absolutely be dealt with by Victoria Police.”

Earlier, dozens of officers created a barricade outside the convention centre to allow attendees to enter the Land Forces International Land Defence Exposition. Senior international military, defence, government, scientific and industry delegations from around the world were among the hundreds expected.

Antiwar protesters flanked the barrier on both sides, throwing eggs and water and yelling “war pigs” or “get a real job” as attendees walked along Yarra Promenade.

“Shame on you, how does it feel to murder babies?” protesters yelled in unison.

As the protest moved to the Spencer Street Bridge about 9.15am, wheelie bins were set on fire and thrown towards police. With tensions flaring, heavily armed police set off canisters of tear gas and used pepper spray foam to control the crowd.

There were loud bangs as the gas was released on the bridge, while protesters chanted “this is not a police state”.

Dozens of protesters and attendees were treated after being hit by sprays and gas. There were also several arrests.

Disrupt Land Forces spokeswoman Jasmine Duff said the protest was triggered by the government’s stance on the conflict in the Middle East.

“We’re protesting to stand up for all those who have been killed by the type of weapons on display at the convention,” she said.

“Many of the weapons inside the convention centre are advertised as battle-tested. In the context of Israeli weapons firms, which are present, this means tested through killing civilians in Gaza.”

Greens MP Gabrielle de Vietri, who was among the protesters, said earlier the state government should have cancelled the “deadly weapons expo”.

But Allan said conflict overseas “should not be turned into an opportunity to drive conflict on the streets of Melbourne”.

“You’re not elected to run off and join political stunts for your own divisive political purposes,” she said on Wednesday.

Victoria Police expected Wednesday’s protest to be its biggest operation in more than two decades as it deployed officers from around the state to the convention centre. Its costs were expected to be as high as $15 million.

State Police Minister Anthony Carbines thanked officers for their support as he issued a warning to protesters.

“There’s nothing but disrespect being shown from some protesters who want to behave like idiots and police will deal with you,” he said.

Opposition police spokesman Brad Battin said he was concerned for the safety of officers.

“We’ve seen these kind of protests get out of hand when violence happens in the street,” he said.

The Land Forces International Land Defence Exposition runs until Friday.

-with AAP