Sections of Boston that lost power because of a smoky electrical transformer fire are slowly recovering but problems are expected to persist for most of the day.

The fire was reported at about 6:30 p.m. Tuesday and knocked out power to about 21,000 homes and businesses.

About 8,000 of those customers had power by Wednesday morning, but many traffic lights and other road signals were still out.

Police say the commute has so far been smooth thanks to a heavy police presence and motorists heeding warnings to stay away from the area.

An NStar spokeswoman says the utility is bringing in generators and hopes to have power back to everyone by Wednesday evening.

The Prudential Tunnel on the Massachusetts Turnpike closed, but reopened Wednesday morning.

Two subway stops in the area remain closed.