MIDDLESEX CENTRE, ONT. -- London police are investigating a suspicious apartment fire that left one person with serious injuries.

The London Fire Department responded to a fire at an apartment building on King Edward Avenue Wednesday afternoon.

One person had to be extricated by firefighters after the fire broke out in the three-storey walk-up shortly before 4 p.m.

The resident was treated on scene by paramedics before being taken to hospital where they remain in serious condition.

The fire appears to have started in a main floor apartment and extensive smoke could be seen coming from the lower levels of the building.

However, fire officials say the entire building was filled with smoke, making the evacuation difficult especially for residents with disabilities.

A London transit bus was brought in to provide shelter for displaced residents.

The fire remains under investigation with the LPS Street Crime Unit involved. There is no damage estimate at this time. 

- With files from CTV News London's Marek Sutherland