Residents of about 500 houses in Takoma Park and Chillum were evacuated yesterday following a chain-reaction of explosions and fires that damaged 13 homes.

Officials gave an all-clear last night for more than 2,000 residents who live in the affected 12 square blocks of Prince George's and Montgomery counties to return to their homes. Officials said the blasts were caused by vapors that seeped into the homes from a flammable, petroleum-based liquid that infiltrated sanitary sewer lines.

About 240 homes were without heat last night, according to Washington Gas Light Co. spokesman Paul Young.

Young said additional time was needed as an extra safety precaution before restoring natural gas service that was cut off earlier in the day. "We will have about 35 men out there around 7 a.m. to restore service," Young said early this morning.

The only injury during the day-long drama resulted from smoke inhalation to a man who fled to his rooftop after fire broke out in his home on Ray Road in Chillum. Four of the house fires were serious, with damage set at $25,000 to one home in Chillum.

Officials planned to sample the air in the sewer lines throughout the night. Tests were to be resumed today on the source of the vapors. At first, officials blamed gasoline, but later said they could not identify the liquid.

Families displaced by the fires were offered shelter by the Prince George's County government at the New Hampshire Motor Lodge on New Hampshire Avenue.

Before residents were allowed to re-enter their homes, they were given printed instructions that advised them to sniff for gas funes or unusual odors before turning on lights or striking matches. They were also advised to flush toilets and run water through drains to create a water trap that would prevent seepage of fumes into their houses. Emergency teams were posted at five locations in the area to assist residents who had concerns about conditions.

Chief Warren E. Isman, director of the Montgomery County department of fire and rescue services, said at least 1,000 gallons of the petroleum-based liquid traveled with water through the sewer line, allowing fumes to seep into the basement of homes through storm drain traps in its path. Officials speculated that the fires and explosions were ignited either by pilot lights in basement heating units or by electrical sparks.

The liquid had not been identified nor had its source been pinpointed last night. Investigators ruled out the possibility that the explosions were caused by natural gas leakage, according to a spokesman for the National Transportation Safety Board. NTSB chairman James Burnett and two hazardous materials investigators were at the scene.

Workers from the Washington Suburban Sanitary Commission, which operates the sewer system, and from fire departments used large fans to suck the fumes from the sewer lines. Fire officials using bullhorns and police officers in cruisers drove through an eight-square block area of Takoma Park and a four-square block area of Chillum, rousting residents along the narrow, tree-lined streets of those normally tranquil neighborhoods.

"They were banging down doors and windows of some houses. I was worried," said Brian Silber, 32, who owns a house at 6510 2nd Ave., Takoma Park. Other residents said they worried about the security of their household belongings after police and firemen told them to leave their homes, with the doors and windows open to vent the fumes. Police said they continuously patrolled the evacuated neighborhoods to deter looting, and none was reporting.

"Usually I don't go out of my house leaving the door wide open," said Patricia Cartright, who lives at 65l6 Alleghany Ave.

The first explosion was reported at 10:20 a.m., apparently originating in the basement of a frame bungalow at 6801 Westmoreland Ave., in the Montgomery County section of Takoma Park, fire officials said.

The explosion "sounded like a bomb in the basement," said Arthur McDonald, the owner. "I tried to get down there but the fire was too much. I have three dogs down there. They're still there," McDonald said.

Three more incidents of fire or explosion were reported on Westmoreland Avenue followed by four more in homes a block north on Alleghany Avenue and another in the 6600 block of Cockerill Street, according to the fire department.

Incidents of fire or explosions were then reported at three homes on Ray Road in Chillum. The final incident was reported at 1:30 p.m. in the 6400 block of Knollwood Drive in Chillum.

As the incidents were reported, officials from the Montgomery County Hazardous Incident Response Team frantically pored over maps of the sewer line routes, charting it out of Montgomery County into Prince George's where it crossed New Hampshire Avenue toward Ray Road.

At the corner of Alleghany and 2nd avenues, firefighting equipment jammed the streets where fires were burning in three houses. In one of the houses, at 6507 Alleghany Ave., an explosion occurred in the basement while a neighbor was in the house using a telephone to report a fire in her house.

Many of those evacuated, including students from Ridgecrest Elementary School, took shelter at Takoma Park municipal center and Nicholas Orem and Takoma Park junior high schools. When the all-clear was given, many returned home on Montgomery County Ride-On buses and school buses. Residents from the Chillum area in Prince George's County were the last to get permission to return, because a high level of fumes lingered there, according to officials from the fire department and the Washington Suburban Sanitary Commission.