The unidentified driver of the pickup was the only person injured in the mishap. His injuries were not serious, police said.

By noon Tuesday, the spill was cleaned up and the residents were allowed back into their homes.

Assistant Fire Chief Dominick Carfi said that one gallon of muriatic acid spilled on the street, along with several pounds of granular chlorine. "The two chemicals don't mix together well," he said. "If they did, it would certainly cause respiratory problems."

The accident took place at Beach and Maple streets, one block west of East Main Street and one block north of Barnum Avenue. The area already is congested by the continuing work to install the 345-kilovolt power line from Middletown to Norwalk, which will run beneath Barnum Avenue.

Carfi said the Fairfield County Hazardous Incident Response Team was called to the scene, along with a crew from the state Department of Environmental Protection. Also assisting in the cleanup was Connecticut Tank Removal of Bridgeport, a private contractor frequently called to help out with hazmat conditions.

"We had a vapor plume of acid, which has since dissipated," Carfi said at the scene. "After the chemicals are cleaned up, they'll neutralize the acid."

Carfi said that the homes that had to be evacuated were on Beach Street between Jane and Barnum streets. Most of the buildings in that neighborhood are two-family homes. The injured driver was taken to Bridgeport Hospital for treatment, Carfi said.

The white pickup turned over on its left side on Beach Street, about 40 feet south of the Maple Street intersection. Police, who were not allowing anyone to get within a block of the accident, roped off the immediate area with yellow plastic caution tape. A Fire Department command post was set up at Maple and East Main streets.

Many of the families who were asked to leave their homes milled about the area, with nowhere in particular to go on the sunny, sultry morning.

"Around 6 a.m. my dad heard banging on the door," said Akeela Gregory, 18, of 143 Beach St., who was with her 4-year-old sister. "He went to answer it. They ran upstairs and told us we had to evacuate, so I grabbed my little sister." "It happened at about 5:30 in the morning," said Joanne Moreno of 163 Beach St. "I'm the one who called the police. The pickup truck somehow got involved with a little compact on the corner of Beach and Maple, but I don't think the compact was damaged. It's not there now. But he did hit a minivan that was parked."

The toppled truck had "Signature Pools" painted on its side. No one from Signature Pools in Norwalk could be reached for comment.