Power outages brought business to a standstill in a large part of Chicago's Loop today, forcing the evacuation of tens of thousands of office workers from darkened skyscrapers and the early closing of the nation's largest futures market, the Chicago Board of Trade.

The affected areas included banks and exchanges in the city's financial center as well as police headquarters, the high-rise Dirksen Federal Building, DePaul University's Loop campus and downtown hotels, including the huge Chicago Hilton and Towers.

Minutes before a "controlled outage" was initiated at 1:45 p.m. by Commonwealth Edison to reduce the impact of an earlier transformer failure, firefighters with bullhorns advised office workers to evacuate the high-rise buildings before the power shutdown ended elevator service.

Some workers stuck in the buildings when the power went out walked down darkened stairways with the aid of firefighters and police. Commuters jammed the city's expressways when they headed home early. The Chicago Transit Authority's elevated trains continued to run because they operate on a separate electrical grid.

A visibly angry Mayor Richard M. Daley told a news conference that Commonwealth Edison has had major unattended infrastructure problems for years and should be taken to "ground zero" and rebuilt with new management. He said "the chickens have come to roost" because the utility had failed to adequately maintain and upgrade its power distribution system.

"We're sick and tired of it. They better change, and if not, the consumers will change it," Daley huffed. He accused the utility's management of failing to notify the police and fire departments of the transformer problems, thereby "putting people in jeopardy, their health and safety."

Commonwealth Edison Chairman John Rowe later said he had told Daley, "No excuse, sir," and said he agreed that rebuilding the utility was needed. "You cannot call this a run of bad luck or a coincidence. The mayor is absolutely right. . . . We must meet a higher level of performance," Rowe said. He said rebuilding the power distribution system could cost hundreds of millions of dollars but vowed to bring in new management to get the job done.

Utility officials said 2,300 customers lost electrical power this morning after three of four transformers at a substation went off line. The first failed Aug. 5, another burned out Wednesday night and the third failed today, prompting the utility to initiate a "controlled outage" in the Loop to lighten the load on the local grid and avert a ripple effect of blown transformers citywide.

Firefighters and utility crews hosed down the one overheated transformer in the near North Side substation and got it functioning again about an hour after the planned outage began.

As a result, electricity to one of the two affected areas was restored at 3:04 p.m., while repair crews were attempting to restore two high-voltage cables that had failed, utility officials said. Power to a larger, 30-square-block area of the South Loop was restored about 11 hours after electricity went off there this morning.