Confusion descended on downtown Thursday when a series of mechanical problems darkened hundreds of office buildings, snarled traffic and skidded the Board of Trade to a halt.
Thousands of commuters left early for the day when power was lost, pushing the rush hour forward by hours. Police struggled to keep cars moving through darkened intersections.
Extra buses were called in to handle the crush and generators were set up in Union Station, which handles Amtrak and suburban commuter trains.
Paul McCoy, vice president of Commonwealth Edison, said cables leading from two transformers failed Wednesday night and Thursday morning at a substation, leaving 2,300 customers without power.
The utility was forced later Thursday to shut down power to another 670 business customers after a transformer at another substation overheated. That transformer had been helping to keep up with demand at the defective station.
Power to all customers was restored Thursday night.
An angry Mayor Richard M. Daley said Commonwealth Edison has had major unattended infrastructure problems for years and should be taken to "ground zero" and rebuilt with new management.
ComEd, which has more than 3.4-million customers in northern Illinois, came under fire last month when nearly 100,000 customers lost power during a deadly heat wave.