In addition to the section of Light Street closed for repairs, the block between Redwood and Baltimore streets was closed and being used as a staging area for the construction work.
J. Kirby Fowler, president of the Downtown Partnership of Baltimore, said business owners in the area were "taking it in stride, and they recognize the city is doing all it can." He noted that most of downtown is open for business and walk-able.
The closed blocks of Light Street are near the heart of downtown and contain businesses that include a hotel and one of the city's tallest office buildings, housing the law firm of Miles and Stockbridge.
At least two buildings on the west side of Light Street between Redwood and Lombard lost water service and were closed Tuesday. The McDonald's at the southeast corner of Baltimore and Light streets also lost water service and was closed.
The buildings on the west side of Light Street house a Royal Farms store, a 7-Eleven and a branch of Susquehanna Bank, as well as upper-level offices. Kocher said water pressure was low at another commercial building, 10 Light Street, but it was open Tuesday.
Chief Kevin Cartwright, a Fire Department spokesman, said fire crews had helped businesses pump water out of their basements Monday.
The city's on-call contractor began repair work Tuesday morning on Light Street. Kocher said workers would be excavating around the site of the break and establishing temporary lines to buildings that lost water service. Officials said water service would be restored to buildings Thursday.
Meanwhile, a second water main break about 2 a.m. Tuesday forced the city to close Fleet Street between Bond and Caroline streets. Repair work in Fells Point was completed Tuesday evening, Kocher said. The rupture damaged a 6-inch water main, he said. Foxx said the Fells Point break was not related to the Light Street break.
"It's not uncommon to have about two or three water main breaks a day," Foxx said.
There were no reports of the water main break affecting water supply outside the city, though Baltimore supplies drinking water to 1.8 million area residents via three reservoirs in Baltimore and Carroll counties, treating the raw water at filtration plants at Montebello in East Baltimore and Ashburton in West Baltimore.
The system isn't quite as old as Baltimore, but it dates to the mid-19th century, when the city bought a 50-year-old private water company and formed its own water department.
Work began shortly afterward on Lake Roland dam across the Jones Falls, and the system kept growing over the next century, with the construction of three large reservoirs. To safeguard people against water-borne diseases, the city began treating its supply with chlorine in 1910, and the first filtration plant followed in 1915.
Baltimore Sun reporters Luke Broadwater, Edward Gunts and Timothy B. Wheeler contributed to this article.