Officers swarmed on the man just after 4pm following reports he was seen driving erratically in a black sedan near the intersection of Wellington Parade and Powlett Street in the CBD, Victoria Police said.

Witness Alexander Hipwell estimated at least 15 police vehicles swarmed the scene, including critical incident response units.

Mr Hipwell was sitting inside the Laurent Boulangerie Patisserie at Wellington Parade-Powlett Street intersection when he saw heavily armed officers drag a Caucasian man from a stationary black Toyota Aurion.

"I watched while the police officer walked up to the car with a handgun in hand. He had it pointed at the driver through the passenger windscreen," he said.

"Another officer approached with what looked like a machine gun. Another with a baton approached on the other side of the car."

"They pulled him out of the car and he was struggling a bit."

He said the police pulled the man onto the footpath and handcuffed him before they began to search his car.

"[They] opened the boot and started pulling out what seemed to be old quilts and clothes and a few cushion looking things," Mr Hipwell said.

"I didn't see them take anything from the car though. They put it all back in the boot and were searching the interior of the car."

"One of the policeman, as I was leaving the area, had a paper bag and I watched him open it and he actually pulled out some football sneakers, soccer boots."

Caller to radio station 3AW, Andrew said the black sedan rammed into the back of a police divvy van as the officers tried to intercept the vehicle.

"He tried to get around and rammed it," he said. "The police all jumped out guns drawn."

A 34-year-old St Albans man was arrested at the scene and is currently in police custody, Leading Senior Constable Natalie Dean said.

He is yet to be charged and will undergo a mental health assessment.

Anyone with information, or who may have dashcam footage, is urged to contact Crime Stoppers on 1800 333 000.