CHICAGO, NOV. 25 -- Mayor Harold Washington, the first black mayor of Chicago, died today after suffering a heart attack at his desk in City Hall.

Washington, who had said frequently that he intended to be mayor for the next 20 years, collapsed in his fifth-floor office at 11:01 a.m. during a meeting with his press secretary, Alton Miller.

He was rushed to Northwestern Memorial Hospital, 20 blocks away, where he died 2 1/2 hours later without regaining consciousness.

Washington, a former congressman and state legislator, became a symbol of rising black political power and expectations when he took control of the nation's third-largest city in 1983 after one of the country's most racially divisive campaigns.

Washington, 65, apparently had no history of heart trouble but had complained of fatigue and a chest virus in recent days.

His death stunned the city and is expected to set off a long and bitter political battle. It came just seven months after Washington had won a second term over his two longtime political rivals, former mayor Jane Byrne and former Cook County Democratic chairman Edward R. Vrdolyak, and took undisputed control of city government.

Washington left no heir apparent. And jockeying over who will ultimately succeed him began almost before he was officially pronounced dead at 1:36 p.m.

"Politics is the number-one game in this town," said Alderman Richard Mell. "Unfortunately, it's being played even now."

Alderman David Orr, the city's vice mayor, will serve as interim mayor until the city council elects an acting mayor from its membership, probably early next week. The acting mayor will serve until a special election is held.

But there apparently was disagreement even in Washington's inner circle about how long an acting mayor will serve and when a special election will be held. Chief city legal counsel Judson Miner was shunted aside at a hospital news conference when he said an acting mayor would serve at least until 1989 and perhaps until 1991 before facing the electorate.

Alderman Timothy C. Evans, Washington's floor leader in the city council and his chief political spokesman, is the early favorite for acting mayor. He took a prominent role in the news conference announcing the mayor's death.

Expressions of praise and condolences poured in from around the world.

Sen. Edward M. Kennedy (D-Mass.) called Washington's death "a tragedy for civil rights." New York Mayor Edward I. Koch (D) said, "His support was broad-based among blacks and whites. And I think that in the future in Chicago, the race of the candidate will no longer be a factor."

Democratic presidential candidate Jesse L. Jackson, an ally and sometime rival of Washington, who announced he was cutting short a trip to the Persian Gulf and returning home to Chicago later changed his mind and said he will continue on to Bahrain, correspondent Patrick E. Tyler reported from Kuwait.

"There is no doubt that when a tree of Harold's magnitude falls, the forest will be full of sorrow," Jackson said, adding, "This is one of the saddest days of my life."

Washington had seemed robust in recent public appearances and had attended a ground-breaking ceremony on Chicago's South Side early this morning. He had returned to his City Hall office and was talking to press secretary Miller about his pending city budget when he collapsed.

"He suddenly slumped to one side," Miller said. "I thought he was trying to pick something up off the floor like a pen. I quickly realized it was something much more serious."

City bodyguards loosened the mayor's tie and began cardiopulmonary resuscitation almost immediately. Washington was in "full cardiac arrest" when he arrived at Northwestern Memorial Hospital at 11:30 a.m., officials said. The mayor's breathing and heart beat returned when he was placed in what was described as a "state of the art" pulmonary machine.

But doctors found that Washington was "brain dead," said Dr. John Sanders, the hospital's chief of staff. "After a period of several hours with no neurologic function, it was our decision to terminate the support system."

The decision, he said, was made after consulting with members of Washington's family, including his longtime fiancee, Mary Ella Smith, who was in Washington's hospital room when he was pronounced dead.

Dozens of Washington's friends, supporters and relatives had gathered in the hospital emergency room. Hundreds of other people gathered outside the hospital, located in one of the city's most prosperous areas.

One woman lay on the cold, wet grass, praying, "Lord, help him. Lord, help us."

"It's so strange. Just like that, it's all over," said another woman, sobbing in the street.

At City Hall, the flags were lowered to half staff, and the city council held a brief prayer meeting.

"I can't hardly believe it. It seems unreal," said police officer Curtis Jones, one of the mayor's bodyguards. "He came in a little after 10. He was walking fast, jolly, smiling, speaking with people as he came in, just like an ordinary day. It's a shock this happened."

"This is a loss for the whole black community," said Darryl Broomfield, another city worker. "I am deeply shocked and saddened by the loss of a great man."

Washington was a hero in much of Chicago's black community, a symbol of racial pride there. He was elected 4 1/2 years ago as a political reformer who vowed to cleanse the city's corrupt political machine.

In his 1983 inauguration speech, he said, "I hope someday to be remembered by history as the mayor who cared about people and who was, above all, fair -- the mayor who helped, really helped to heal our wounds and stood watch while the city and its people answered the greatest challenge in more than a century, who saw the city renewed."

His first term, however, was marred by battles with members of the regular Democratic machine in what became known as "Council Wars." Washington eventually won the wars -- and reelection, collecting 96 percent of the black vote. His archrival, Vrdolyak, leader of white opposition forces in the City Council, recently became a Republican, an acknowledgment of Washington's control over the city's Democratic Party.

In winning reelection this year, Washington became the first Chicago mayor to serve more than one term since the legendary Mayor Richard J. Daley, who was serving his sixth term when he died of a massive heart attack 11 years ago. Then the big-city machine he had ruled began to crumble.

Washington was born April 15, 1922, in Cook County Hospital and lived in the city's South Side all his life. He represented Hyde Park, a racially mixed area that includes the University of Chicago, in the state legislature and Congress. He was graduated from Roosevelt College and Northwestern University law school.

Washington began his political career as a foot soldier in the Daley organization but broke with the machine. He served in the state legislature as a representative and a senator from 1965 to 1980, when he was elected to the U.S. House.

Washington ran unsuccessfully for mayor in 1977 against then-Mayor Michael Bilandic (D), picked by the city council as Daley's successor. In 1983, he upset Mayor Byrne and State's Attorney Richard M. Daley, a son of the late mayor Daley, in the Democratic mayoral primary. He beat Republican Bernard Epton in one of the most racially tinged elections in recent history.

Earlier this year, Washington again defeated Byrne in the Democratic primary. He went on to beat third-party candidate Vrdolyak and Republican Donald H. Haider in the general election.

Ironically, by the time of his death, Washington was being compared to Mayor Daley as a political broker. Just last week, he flexed his muscle to keep the Democratic Party from slating former mayor Byrne, who had tried to court him, as a candidate for Cook County clerk. He also had recently proposed a major increase in property taxes.

Washington's slogan was "Chicago works together." Mayor Daley's slogan was "Chicago, the city that works."

Friends and supporters conceded today that Washington had not accomplished the reforms that had been his goals. "His death was very untimely. Things were just getting into place," said Leon Finney, president of the Woodlawn Organization, a grass-roots group. "They weren't quite there, but they were on the way."

As one of the nation's most powerful black elected officials, Washington had begun to take an increasingly prominent role in national urban policy debates and national Democratic Party politics as he solidified his home base.

"Mayors all over the country respected him {Washington}. This is not the end; this is the beginning of a legacy," said Alderman Eugene Sawyer, president pro tem of the city council and one of the people mentioned as a possible successor.

The power vacuum Washington left, however, was in Chicago. Jackson recognized this from the Persian Gulf. He predicted "a tremendous battle" and said, "I hope the coalition {that elected Washington} pulls together."

Under city law, Orr became acting mayor upon Washington's death. He is a liberal, former professor of history from Rogers Park, a predominantly white area. He is not considered a mayoral candidate. At a news conference announcing Washington's death, Evans, first elected to the council in 1973, and Ernest Barefield, Washington's chief of staff, put themselves forward as the people in charge. "In the great mayor's memory, we will all strive to do everything we can to pull together," Evans said. Barefield said Washington had never spoken to him about a successor. "The mayor always had a great expectation that he would be mayor for 20 years," he said.

Washington's national stature was reflected in the rush of praise and eulogies from across the nation.

President Reagan, in a statement issued from his California ranch, said, "Harold Washington had a distinguished career in public service. . . . He was a dedicated and outspoken leader who guided one of our nation's largest cities through the 1980s."

Illinois Sen. Paul Simon, who had known Washington for 30 years, said, "When I recall Harold, I think of words and phrases like independence, compassion, genuine reformer, great sense of humor."

Los Angeles Mayor Tom Bradley said, "The loss of his leadership and humanity is a great loss to Chicago and the nation."

Mayors Coleman Young of Detroit, W. Wilson Goode of Philadelphia and Richard Hatcher of Gary, Ind., also praised Washington. "The sudden death of Chicago Mayor Harold Washington was a great blow. . . across this land," Young said.

Illinois Gov. James R. Thompson (R) ordered the state's flags at half staff, saying, "The city of Chicago and the state of Illinois have lost a dedicated public servant today."

Services for Washington are scheduled to be held Monday.

Special correspondent Janice Kramer contributed to this report.