WASHINGTON--Mayor Marion Barry, his last years in office marred by accusations of drug use, was arrested on a drug charge last night in what federal authorities described as "an ongoing public corruption probe."

The arrest at a downtown hotel came just three days before Barry planned to formally announce that he would seek re-election to a fourth term and a day before long-time Barry associate Charles Lewis was scheduled for sentencing on drug charges.

Lewis testified in federal court that he provided cocaine for the mayor, but it was not clear whether last night's arrest was connected to the Lewis investigation.

Barry has steadfastly denied that he used drugs.

The U.S. attorney's office said Barry would appear before a federal magistrate on unspecified drug charges today. The office did not say where Barry would be held overnight.

A federal law enforcement source who asked not to be identified said the arrest was the result of a "sting" operation in which FBI agents kept under observation a prearranged meeting between the mayor and an unidentified friend in a downtown hotel, the Vista International, about six blocks from the White House.

Barry, 53, was arrested at the hotel shortly after 8 p.m. as a result of an undercover operation conducted jointly by the city police department's internal affairs division and the FBI, according to a statement by U.S. Attorney Jay Stephens and Thomas DuHadway, special agent in charge of the FBI's Washington office. There was no word of additional arrests.

City police officers barred access to upper floors of the hotel to all but guests. A dozen or so teen-agers mixed with reporters, discussing the arrest, outside.

A city official who asked not to be identified said Barry had no events on his schedule at the hotel last night.

First word of the arrest was broad-cast shortly after 10 p.m. by television station WRC, which said the mayor attempted to buy cocaine on the seventh floor.

The mayor, the city's second under home rule, was led away by under-cover police officers and FBI agents, the station said, and his driver and bodyguard were led away separately.

Barry was taken to the FBI's national headquarters, said federal sources who asked not to be identified. That building has no detention cells.

Barry, who has repeatedly won re-election with large majorities has lost support over the past four years because of allegations of drug use and corruption convictions against several members of his administration.

Rev. Jesse Jackson fueled speculation that he would challenge Barry when he moved to Washington last year. However, Jackson has said repeatedly that he would not run against Barry. Jackson said through a spokesperson that he would have no comment on the arrest.

Drug use among the city's young men is blamed for a sharply rising murder rate--a record 438 homicides were committed last year against 369 the year before.

Top city officials were meeting late last night to assess the arrest. City Council Chair David Clarke, who is running for mayor, called the arrest "a tragic moment in our city's history." Councillor Charlene Jarvis, another mayoral candidate, said the arrest "changes the political landscape of the city."

Although rumors about Barry and drugs had surfaced throughout the last half of his tenure as mayor, no charges had been brought before yesterday.

The accusations became even more widespread after the arrest of Lewis.

Barry was visiting Lewis' hotel room in December 1988 when city police were sent to investigate a report that Lewis was selling drugs. When the officers found out Barry was present, they abandoned their mission in an episode not yet fully explained.

Lewis pleaded guilty to one count of conspiracy to distribute cocaine in Washington and was scheduled to be sentenced on that charge today. Lewis also has been convicted of selling drugs in the Virgin Islands.

In November, WUSA-TV said a local physician had reported to police that Barry had been treated for a drug overdose in 1983. The then-police chief later reported that investigators were unable to confirm that report.

Karen Johnson, a former city worker with whom Barry eventually admitted a romance, was convicted on various drug charges in 1984. She served several months in prison for refusing to testify about Barry.