A man who was acting erratically with a trolley full of petrol cans and potentially hazardous material, sending part of Melbourne's CBD into lockdown, is in hospital under police guard.
Police later confirmed no dangerous chemicals were found, but said reports that the man had a cylinder caused the alarm.
"I can't tell you what was contained in that cylinder, but it was sighted and that initiated that police response," Acting Senior Sergeant Bonnie Heazlewood told reporters.
Investigators arrested a 52-year-old Kurunjang man after negotiating with him for about 45 minutes at a building on the corner of William and Bourke streets early on Tuesday.
A witness told AAP a man appeared to be pushing a trolley containing chemicals.
Another witness said the man smashed tiles on the ground and claimed to be in possession of asbestos.
Police initially reported that the man had petrol cans, but they were later found to contain only water.
"There's been no property damage and no injury to any persons at this stage," Acting Senior Sergeant Heazlewood said.
"We took the initial response for the public's safety, which resulted in a temporary lockdown of the city.
"It's an isolated incident. It wasn't something that was premeditated."
Fire Rescue Victoria said the incident was declared under control at 8.15am after firefighters determined there was no flammable risk.
Witnesses Sam Di Palma and Daniel Safstrom said they were getting a coffee when they saw a man in the building with a trolley and officers trying to talk to him.
"A bloke (was) behind a counter with a trolley full of chemicals or explosives," Mr Di Palma told AAP.
Mr Safstrom said the police "sent one of their guys in to sit down and negotiate, so they both grabbed a chair".
"I'm a bit shocked to be honest. It's something different," he said.
Another witness, Abraham Elosman, said the incident gave him flashbacks to the Bourke Street Mall attack in January 2017.
"I thought maybe they were doing an exercise or something. I didn't think there was gonna be this big issue," he told AAP.
"But when I saw the shields and the fire brigade come in, I was a bit worried. I thought 'I hope it will be OK'."
At 11am police said they had not spoken to the man under arrest, so they could not confirm whether the incident was being treated as a criminal matter.
The building houses offices for CGU Insurance, the Commonwealth Director of Public Prosecutions, Court Services Victoria, and legal firms including Gordon Legal and Baker McKenzie.