A specutacular three-alarm fire swept through the grandstand of the abandoned Marlboro race track late last night, destroying the 50-year-old Prince George's landmark. Investigators said they believe the blaze had been deliberately set.

The track, located on Maryland Rte. 4 near the county seat of Upper Marlboro, figured prominently in the political corruption trial of former Maryland governor Marvin Mandel. The dilapidatedf, clapboard grandstand and the track itself had just been sold to Prince George's County for $1.1 million for use as a public equestrian center and parking lot. The track has been closed since 1972.

Nearly 100 firemen battled the fire for about three hours. Fire officials said the blaze, which could be seen from 10 miles away, was brought under control shortly after midnight.

At least four firefighters suffered minor injuries in the blaze, but none required hospitalization, officials said.

The adjoining clubhouse and stables apparently were not damaged, but county officials said there was no practical reason now to save them. The track, which had no full-time security guard, had been used primarily in recent years as a parking lot.

The grandstand was considered not safe and the area had been closed off to keep out intruders.

The roof of the grandstand was tilted back like an accordian by the fire, and its metal beams resembled "twisted paperclips," one witness said.

Fire officials estimated the damage at $200,000.

The race track was once the pride of the county, a posh gathering place with record numbers of bets placed.

By 1970, it began losing its following to bigger tracks such as Bowie's mile-long course. A year later, Marlboro Associates, a partnership that secretly included friends of Mandel, bought the track. Mandel then used his influence to get the 1972 General Assembly to give it more racing dates, which increased the track's values.

Shortly afterward, Mandel's friends sold the track at a hefty profit. Mandel and five codefendants subsequently were convicted on political corruption charges. Last spring, the U.S. Supreme Court refused to review the convictions and the former governor currently is serving his sentence at Eglin Air Force Base in Florida.

Prince George's County officials purchased the track recently from the Southern Maryland Agricultural Association, part of a company that also operates the Bowie race track.

County officials wanted to convert the race course area into an equestrian center, and use the rest of the land as a parking area. However, County Executive Lawrence J. Hogan said last night the fire dashed any hopes of refurbishing the track.

County authorities declined to speculate last night on why the fire might have been set. Investigators said they had not been able to determine exactly how it was started.