Heavily armed police have swooped on Melbourne's sports precinct, which is in lockdown during the hunt for the gunman.

The man, of Spanish appearance and in his mid-20s, is believed to be armed with a shotgun and witnesses say that he has been spotted in the area around Melbourne's sports precinct

The hunt follows a dramatic high-speed pursuit on the Monash Freeway through several suburbs this morning.

Police have just released a description of the man who is described as in his mid-20s, of Spanish appearance, with straight dark hair, wearing a dark jacket with red or orange t-shirt underneath and silver tracksuit pants.

He is considered highly dangerous and authorities have warned people not to approach the man.

Police have warned the public to stay away from the Melbourne Cricket Ground and Olympic Park precinct as heavily-armed police wearing bullet-proof vests were seen entering the sports precinct.

The Melbourne Cricket Ground and the Rod Laver Arena have been closed.

Several roads are closed to traffic and pedestrians on Olympic Boulevard, Batman Avenue, Punt Road, Wellington Parade and Brunton Avenue.