TAUNTON, MASS. ? City officials on Tuesday closed the downtown business district, canceled school and evacuated nearly 2,000 residents as fear mounted that an old wooden dam could burst under pressure from a rain-swollen river.

The Whittenton Mill Dam, built in 1832, is deteriorating and could unleash a wall of water up to six feet high, Mayor Robert G. Nunes said Tuesday.

“The city of Taunton is still in a state of emergency,” he said. “If the dam goes, it will create massive flooding along the Mill River and into the downtown area.”

The 12-foot-high dam began to buckle about 2 a.m. Tuesday. So far this month, 11 1/2 inches of rain have fallen on Taunton, a city about 35 miles south of Boston. Officials said at least one timber column had washed away, allowing water to leak through and under the dam.

By early Tuesday morning, an emergency response team had converged on this city of 56,000. The gymnasium at Taunton High School was converted into a Red Cross shelter. Massachusetts Gov. Mitt Romney visited the city Tuesday morning. Nunes said that the state’s Democratic senators, Edward M. Kennedy and John F. Kerry, and Democratic Rep. Barney Frank had called to promise federal assistance if necessary.

“Obviously, a situation like this is one of the largest threats we have faced in a very long time,” the mayor said.

Bob Nadeau, regional director for the Massachusetts Emergency Management Agency, said the voluntary evacuation included more than 200 homes and apartment buildings along the river, as well as the closing of Taunton’s commercial district.

“You evacuate when you have to,” Nadeau said. “If anything was learned from New Orleans and more recently in New Hampshire it is that if people do not evacuate, you lose lives.”