Explosions rocked downtown London last night, sending manhole covers flying and columns of flames shooting up from the sewers. No one was hurt and damage appeared to be minimal when an electrical fire sparked explosions and smoke spewed underneath Galleria London and adjacent streets.

Hundreds of people were forced to the streets as the downtown mall and Central Library were evacuated.

Officials with London Hydro were testing for toxic PCBs, which are used in coolant for electrical transformers, but results weren't known last evening.

Firefighters, police and ambulances rushed to the scene shortly after 7 p.m.

Streets were closed and the area evacuated.

"When I got here, there were flames 30 feet high shooting out of the sewer," police Sgt. Peter Mair said.

A police officer narrowly escaped injury when a manhole cover blew into the air just a few feet away.

"It was a pretty scary situation for a while," Mair said.

Four manhole covers were blown several metres into the air, the explosive force of one damaging the bottom of the King Street walkway, police said.

"You can imagine the force," Staff Sgt. Dean Lees said.

Smoke seeped into the Galleria near Rockwater Brewing Company and into the basement of at least one business on the west side of Clarence Street.

The explosions centred near the corner of Clarence and King streets.

Hundreds of movie-goers, library and bar patrons were ordered out of the area bounded by Richmond, Wellington, York and Dundas streets as police set up barricades.

London Hydro and Union Gas crews were at the scene, helping firefighters and investigating the cause.

The electrical fire may have ignited sewer gases, one officer said.

Shortly before 8 p.m., police were told natural gas was no longer a threat, easing concerns. It's not known if the natural gas was shut off.

Mair said the evacuations went smoothly.

"No one was injured. We're really lucky, but we're not over this yet," Mair said.

Dozens of people, many with cars left behind, milled around street corners braving frigid winds and blowing snow, some without coats.

"Now we're stranded," said Leigh-Ann Malcolm, 18, who was in a theatre waiting for the movie Gothica to start when the fire alarm sounded.

"The only buses we can get home are on Dundas and they aren't running."

Security guards blocked entrances to the mall and its parking garage.

A police cruiser equipped with a loudspeaker advised pedestrians to stay away from manhole covers and warned of potential for more explosions.

Police around the perimeter also warned pedestrians to stay clear of sewer manholes.

Shortly after 9 p.m., police allowed Galleria patrons to retrieve their vehicles from the underground parking garage.