At least seven licensed premises across Melbourne have been targeted over the past week, including two on Friday.
Southbank's Soho Bar was torched in the early hours of Friday morning, with the firebugs also attempting to damage the business next door.
A single gunshot was fired at a CBD premises on Lonsdale Street in a drive-by shooting about 10 minutes earlier.
Business owners were co-operating with police and had not reported any extortion attempts, said Chris Murray of Victoria Police's arson and explosives squad.
"There's a lot of motives and theories floating around," Detective Inspector Murray told reporters on Friday.
"It's early days but we are none the wiser as to why these premises are being targeted."
Two boys, aged 16 and 17, were arrested and questioned over the Southbank firebombing.
Det Insp Murray said the teens were not believed to have gang links and could have been responding to the criminal equivalent of AirTasker.
In a method used throughout Victoria's tobacco wars, youths continue to be paid as little as a few hundred dollars to carry out the dirty work of anonymous crooks.
"These young kids are being used as cannon fodder," Det Insp Murray said.
Other recent incidents include an attempted arson on a Lonsdale Street premise on Tuesday and a fire at a South Melbourne venue on Wednesday.
Detectives were also investigating a targeted fire at a South Yarra venue and an attempted arson on a South Melbourne address on Thursday.
Det Insp Murray conceded there was a risk of further attacks in coming days, but stressed police had no intelligence a particular business was in the firing line as patrols were ramped up.
Police have urged the hospitality industry to update their security and called for residents and people visiting the CBD and city's southeast this weekend to report any unusual activity.
"We see young males get out of vehicles in possession of jerry cans at three o'clock in the morning," Det Insp Murray said.
"If you see that, obviously that's something we're interested in."
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