Alexandria's City Hall was set afire early yesterday, not by scorching speeches or incendiary debate, but by what city fire officials said was an apparent arson attempt.

Investigators said three separate fires appear to have been set in the carpeting and a wooden information counter in a reception area immediately inside the 110-year-old building's main entrance off Market Square.

A private company that monitors recently installed smoke detectors in City Hall notified firemen at 5:06 a.m., and the blazes were put out in about 10 minuites, city officials said. Damage was estimted at $3,000.

"I hate to think somebody would try to burn down this old building," said receptionist Sue Billmyer, as she handled incoming cals yesterday at a switchboard set up temporarily on a table in the lobby. The building was erected in the early 1870s after fire destroyed the Alexandria Market House, which once housed city offices.

City Manager Douglas lHarmon quoted fire officials as saying flames about 6 feet high were threatening to ignite the tile ceiling over the receptionist's desk when firefighters arrived.

The building was vacant at the time, but investigators found the door off the square ajar, Harmon said.

Yesterday's incident may have been the second attempt recently to damage the building, which houses some administrative agencies, the city manager's office and City Council chambers. Harmon said officials discovered evidence last week of a small fire under a window outside his office. The matter is under investigation.

One wing of City Hall is undergoing a two-year, $5 million to $6 million renovation to convert space previously used by the state courts into city offices. "You always have worries about fire hazards during reconstruction," said Harmon. He said that concern had led city officials to install the new smoke detector system.

In late July, a fire on the top floor of the federal courthouse, located three blocks from City Hall, gutted the chambers of senior appellate Judge Albert V. Bryan. The FBI later determined the blaze was accidental.