An emergency warning has been downgraded after firefighters contained a blaze which was threatening lives and homes in the shires of Mundaring and Northam, east of Perth.

The warning was for some residents in parts of The Lakes, Copley and Woottating, in the shires of Northam and Mundaring.

At least one vehicle is believed to have been destroyed by the fire.

Around 100 firefighters battled the blaze, which burnt through about 276 hectares.

A bushfire watch and act is in place for people in an area bounded by Great Southern Highway, Horton Road, Chinganning Road, Inkpen Road and Woottating Nature Reserve in parts of The Lakes, Copley and Woottating, in the shires of Northam and Mundaring.

More than 140 homes in the area are without power, with work to restore electricity due to start on Thursday morning.

The blaze started near the intersection of Horton Road and Great Southern Highway in The Lakes.

It was reported to authorities just after 1:00pm on Wednesday.

It is not known what started the blaze. Acacia Prison was initially the emergency fire zone and corrective services staff were in close contact with fire authorities, but no evacuation was necessary.