Fire investigators have classified as arson three blazes yesterday in downtown Washington, including two that forced the evacuation and closing of the north building of Woodward & Lothrop's main store and all of the Hecht Co.'s nearby main store.

Both stores were cleared safely of customers and employes, and officials of both companies said their downtown locations would open as usual this morning.

Another fire, reported at 2:43 p.m., caused evacuation of the International Building at 1319 F St. NW. Damage to the office structure was confined to a ninth-floor hallway and a fourth-floor stairwell and was estimated at $1,500.

Fire Inspector Willie E. Drummond said the fires "all appear to have been very similar in the way that they were set." He said investigators had found evidence at two locations that flammable liquids were used to start the fires.

A witness chased a man from the scene of one of the fires, Drummond said. Police said last night they had made no arrests.

Edwin K. Hoffman, chairman of Woodies, described the blaze in the north building of his downtown store, at 11th and G streets NW, as a "small, smoky fire" that was started in a second-floor storage area for Christmas display materials, near the toy department. The fire set off the area's sprinkler and alarm system at 3:29 p.m. and firemen quickly put it out, Hoffman said.

The fire was limited to the display materials, which were made mostly of plastic and emitted heavy smoke that filled the building, Hoffman said.

"I was over there and I had to get out. It the smoke was pretty bad," Hoffman said. Fire officials estimated damage to merchandise at $10,000 to $20,000. In addition to smoke and water damage, Hoffman said several windows in the building were broken by firefighters as a way to clear the building of smoke more quickly. That damage was expected to total about $100,000.

The fire at Hecht's store at Seventh and F streets NW was reported at 4:12 p.m. and started in a couch and chair, which were in separate parts of the main floor for furniture, according to fire officials.

Edgar Mangiafico, chairman of Hecht's, said the damage was limited to the immediate area of the fire. "We took the precaution of closing the store because of smoke on the fourth floor," Mangiafico said. Drummond estimated damage would exceed $20,000.

Firefighting equipment filled the streets around Hecht's and snarled afternoon rush-hour traffic. Although the 11th and G streets NW entrance to Metro Center was closed for a short time, Metro officials said their operations were not affected by the Woodies fire.