A teenage boy was arrested in dramatic circumstances that left the Melbourne CBD locked down for most of the morning today.

Emergency services were called to the corner of Swanston and Flinders streets just before 8am in response to a vehicle being driven erratically.

Passers-by in the busy area took it upon themselves to try to stop the car.

They threw a hire bike under the car in a successful bid to slow the vehicle.

Police later arrested the person, believed to be a 15-year-old boy from Knoxfield. He currently remains in police custody.

Video from scene shows police negotiation with the man as he paced around the intersection dressed in full body army gear.

“It’s yet to be confirmed, but we believe he has a mental health issue,” police said.

“We’ve got no information he was trying to harm anyone.”

The man, who is yet to be identified, is now in hospital being assessed.

Roads and public transport were shut down for hours.

All roads are now open, and trains and trams resumed running shortly before 11am.

Police said they don’t encourage people to involve themselves in such instances, but added that today’s incident shows the community has a “heightened sense of vigilance”.