Vandals have targeted the heart of Victoria's union movement by spray-painting anti-police and pro-Palestine slogans on the building, demanding the police union be kicked out.
Police are investigating vandalism at Victoria's Trades Hall in Carlton after the building was graffitied about 3.30am on Thursday.
About four people dressed in black clothing and wearing masks were involved in spray-painting anti-police slogans which included "Cops defend genocide" and "Cops out of Trades Hall" along the building's walls.
Pro-Palestine slogans were also sprayed on the building.
An anarchist group that has been targeting politicians' offices over the Israel-Gaza war are believed to be behind the vandalism, Victorian Trades Hall Council secretary Luke Hilakari said.
"They have a view that police shouldn't be part of the Trades Hall Council and that is linked to their frustration about police being at rallies for Palestine," Mr Hilakari told AAP.
There has been an ongoing campaign for the union representing police to be kicked out of Trades Hall with protesters arguing police use their power to oppress marginalised communities.
"Change doesn't come from the end of a spray can, our relationship with the Victorian Police union has only got stronger over time," Mr Hilakari said.
"That won't change."
CCTV footage captured the four carrying a bag and holding a camera believed to be recording the incident which Mr Hilakari branded as a "performative and dumb stunt".
It's the second time in a week Trades Hall has been targeted by the vandals who had earlier tagged anti-police slogans in a bathroom available for the public.
"We don't want to close our doors and we don't want to have more security. We like the idea that this is open space and it's the people's space," Mr Hilakari said.
"But when things like this happen, it makes us think twice about how much access do we give to everybody."
Police Association of Victoria secretary Wayne Gatt dismissed the vandals' demands.
"It appears these vandals have delved into their grab bag of cliched anarchist catchphrases and yelled them onto a wall," he told AAP.
"Bravo. They left out 'Romans go home'."
Premier Jacinta Allan on Thursday said Trades Hall had stood in solidarity with people of all backgrounds and beliefs for more than a century, demanding an end to the vandalism.
"If somehow the message is still being missed: violence in Victoria won't stop violence in the Middle East," she said on social media.
Mr Hilakari thanked the Melbourne City Council's deputy mayor who offered to clean up the graffiti at no expense to Trades Hall.
The incident comes after federal Jewish Labor MP Josh Burns' electoral office was targeted by vandals who smashed windows, spray-painted anti-Zionist slogans and lit fires outside on Wednesday.
