The historic Olney Inn, a Montgomery County dining spot since 1926, was virtually destroyed yesterday in a fire that officials said was arson.

The blaze, which raged for several hours, caused about $500,000 damage to the frame building, which dates to 1835, and its antique contents.

Fire officials said two juveniles were spotted near the inn, at 17801 Georgia Ave. in Olney, shortly before the fire began. Officials believe the fire was started in a pile of leaves at the rear of the building, according to Lt. Leonard King, of the Montgomery County Fire Department.

A developer was to have signed papers yesterday completing his purchase of the inn, which he reportedly planned to integrate into a shopping arcade.

But news of the devasting fire apparently halted settlement proceedings that would have closed the $125 million deal. Owner Harry Simms then rushed to the scene of the fire.

The old farmhouse became an inn in 1926, after Clara May Downey had a flat tire on the property, saw the house and fell in love with it. She started a tea room there that included among its customers a woman boarder with an English maid.

The inn's most famous guest was President Franklin D. Roosevelt. In recent years, it has been considered a fashionable place for Sunday brunch and theater suppers.

Simms, of Silver Spring, was set to sell the inn and about 5 acres around it to Washington real estate developer Robert N. Wolpe, according to Simm's attorney, Hal Lackey.

The inn was the second historic building in Olney to be hit by arson in less than a year. Last May, the Fair Hill Farm was severely damaged by a fire that officials later said was deliberately set.That farm was also the site of a planned shopping center, which fire officials say is now under construction. Fire officials said they did not believe there was any connection between the two fires.

Two Montgomery County firefighters suffered minor injuries during the fire, King said.

Several employes, who were in the building when the fire begin, were evacuated safely. A nearby barn that houses an antique shop was not damaged.

King said the 2 1/2-story building, devoid of any "fire stops" in its walls, burned quickly.