A fire was set at a Falls Church abortion clinic last night, and police arrested a Prince William County man who authorities said had broken into the building.

No one was injured by the small fire at the Commonwealth Women's Clinic, in the 900 block of West Broad Street, which is the scene of regular picketing by antiabortion activists.

James Anthony Mitchell, 38, of Nokesville, was arrested by officers investigating a burglary report. After taking Mitchell into custody, the officers discovered that a fire had been set in a hallway, Falls Church officials said.

Firefighters quickly put out the fire, which caused some smoke and water damage.

Mitchell, who was charged with possession of burglary tools and burglary with intent to commit a felony, was being held without bond in the Arlington County jail.

Police would not say whether they knew of a motive for the fire or whether Mitchell was known to be active in the antiabortion movement. Several activists on both sides of the issue said last night that they were not familiar with his name.

The FBI and the Bureau of Alcohol, Tobacco and Firearms have joined Falls Church authorities in the investigation.

It was the second time in 2 1/2 years that a fire was started at the clinic. In July 1994, rags and other material were set ablaze at the clinic's front door. Only the door was charred, and there was no other damage.

Wayne Codding, operations director for the clinic, did not return telephone messages left at his home last night.

Falls Church Mayor Alan Brangman said he was unaware of any recent threats against the clinic, although "on average, every other Saturday, there are protesters here."

"It's unfortunate {that} every once in a while you're going to find there are people who don't really care about other people's rights and take the law into their own hands, rightly or wrongly," Brangman said.

Heather Amsden, president of the Washington Area Clinic Defense Task Force, said members of her group are at the clinic every Saturday to escort patients into the building.

"Without having people out there 24 hours a day, seven days a week, obviously there are going to be people who are going to go after it," she said. "It could be any clinic anywhere. . . . All clinics are always keeping their eyes open for this kind of activity, but there's not any real way to prevent it."

The Rev. Donald Spitz, of Pro-Life Virginia, said he planned to contact Mitchell and offer whatever support he could.

"The moderate pro-lifers don't accept what we believe," he said. "They would say they would prefer the building would be standing. . . . Other groups are focused more on theory or some other philosophy or something like that. . . . They don't seem to take seriously that babies are dying."

Since 1977, there have been at least 1,700 bombings and arsons committed against clinics across the country, including the bombing of an Atlanta clinic and several bombings of a Tulsa clinic last month. The last bombing on a Washington clinic occurred in 1985 and the most recent arson, before last night, was at the Commonwealth Women's Clinic.

The first sign of trouble at the Falls Church clinic came about 7:30 p.m. when police received a call about someone breaking into the two-story brick structure, city spokeswoman Barbara J. Gordon said.

Police officers were unaware that a fire had just been started when they got inside and arrested a man later identified as Mitchell. Firefighters from Falls Church and Arlington were called to extinguish the blaze.

"There was a little bit of damage in the hallway, but {firefighters} caught it and knocked it down before it extended," Falls Church Vice Mayor David Snyder said.

Snyder and Brangman said quick action by police and firefighters prevented further damage.

"It couldn't have been more than a minute or so when {police and firefighters} got there," Brangman said.