Business halted in a busy section of downtown Baltimore yesterday after one of the city's largest water mains burst Thursday night, dumping hundreds of thousands of gallons of water into two major office buildings and clogging traffic throughout the area.

Utility crews worked to repair damage in the first block of St. Paul Street, where a 20-inch cast-iron pipe ripped open about 10 p.m. Thursday. The cause of the break was not known, but a city official said it might have resulted from ground movement during the seasonal change.

"The ground is always shifting, but a pipe is rigid," said Kurt L. Kocher, spokesman for the city Department of Public Works. "When the ground moves, a pipe has to move as well."

Crews were able to stop the flow of water by 4:30 a.m. yesterday, Kocher said. But by then at least two major buildings had been flooded.

The William Donald Schaefer Tower, at 6 St. Paul St., and the First Union Building, at 7 St. Paul, were closed yesterday. The Schaefer Tower apparently sustained the most damage, with 300,000 to 350,000 gallons of water flooding its parking garage. The First Union Building was also flooded, although the amount of water was uncertain.

"The water levels got so high for a while that the cars in the [Schaefer building] basements were floating," said Nick Sarchiapone, a Baltimore Gas and Electric Co. crew leader who worked at the site all day. Sarchiapone said a half-dozen state vehicles were flooded in the Schaefer Tower's basement parking garage.

At street level, traffic backed up on Fayette and St. Paul streets around the flooded area yesterday evening as a pump spewed muddy water out of the gaping repair hole into the gutter.

City officials said they hoped to finish repair work by today. The portion of St. Paul between Fayette and Baltimore streets was closed yesterday and could remain that way through at least the weekend, officials warned.

Motorists should expect delays during the weekend, especially with two major events scheduled for Ravens Stadium - today's Navy-Notre Dame college football game and tomorrow's NFL Ravens-Cincinnati contest.

St. Paul Street is a major southbound thoroughfare downtown, and motorists are encouraged to use Guilford Avenue and Cathedral Street as alternative routes.

"We do ask for patience," said Adrienne Barnes, spokeswoman for the Baltimore Office of Transportation. "Any kind of traffic interruption is going to have a domino effect on surrounding or adjacent traffic patterns."

This has been a particularly hard year for the city's aging water pipe system.

During the summer, the city recorded more than 113 water main breaks, double the number from last summer, Kocher said. Officials theorized that the intense heat caused the ground to shift and buckle and that the drought caused a redirection of water, stressing the pipes.

The number of ruptured water mains usually increases at this time of year, Kocher added, primarily because of fluctuations in temperature that cause ground movement.

Early yesterday morning, the 1,500 state employees in the Schaefer building were called at home and told not to report to work. Bank employees showed up to find "Closed, Water Main Break" signs on locked doors.

Public works crews, meanwhile, got to work. Using a backhoe to open a 10-foot-wide hole in the street, workers dug to find the leaky water main. Once they located it, crews worked to repair gas, electric and steam lines.

"With a downtown water main break, the most important thing to remember is there are utilities all under the streets," Kocher said. "Within a foot of this water line, there are many electrical cables. And water and electricity just don't mix very well."

Thursday night's rupture sent thousands of gallons of water pouring through electricity conduits into the basement parking garage of the Schaefer building. The water rose to about five feet in a lower level, but by last night only 10 inches remained. Electrical power was shut off to both the Schaefer and First Union buildings as a safety precaution.

Steven F. Noonan, assistant superintendent for the Maryland Department of General Services, which runs the Schaefer building, said the damage to the building appeared to be minimal. He expects it to open by Tuesday when employees return after the Veterans Day holiday.

At the nearby Walgreen's drug store on Fayette Street, employees were unable to develop photos or use the bathrooms because the water was shut off. A few hundred dollars' worth of merchandise was destroyed by the water leak, said assistant manager Ed Heath.

"We've been all right," Heath said. "The construction site has hurt sales, though, because people have been afraid to cross the street to visit us."