CHICAGO, DEC. 1 -- The city council tonight hesitated in picking a successor to the late Harold Washington, Chicago's first black mayor, after thousands of demonstrators gathered at City Hall to protest the front-runner.

Alderman Eugene Sawyer, a black South Side ward boss, had enough votes in the city council to be elected acting mayor, a position that will last until April 1989. But Sawyer apparently developed cold feet as the crowd grew to over 4,000. He repeatedly delayed the city council vote from its scheduled 5:30 p.m. until midnight, and it appeared the council would not vote until Friday.

"Uncle Tom. Uncle Tom. Sawyer. Sawyer," chanted the crowd, which filled the lobby of City Hall and occupied a block of LaSalle Street. "We put you in. We'll put you out."

Sawyer, the council's president pro tem and known for his low-key manner, held a series of meetings during the evening with his chief rival, Alderman Timothy C. Evans, in hopes of gaining his support. He also called in a number of ministers for long prayer sessions.

Sawyer's forces won two test votes tonight. But one of Sawyer's chief lieutenants, Alderman William Beavers, said Sawyer had the votes but not the stomach to be elected acting mayor and had decided to postpone balloting until Friday or "until he gets the support of the black community."

Beavers said he and Sawyer had informally polled voters in their wards during the day and found a tremendous negative reaction to Sawyer's candidacy. "He does not have the support of the black community. You have to be able to go back into your community to be mayor."

The city council session was scheduled to begin at 5:30 p.m. But the gavel convening the session did not sound until almost four hours later. When Sawyer walked into the chamber, he was greeted by loud boos. His knees buckled, and he appeared faint.

The council then recessed, and Sawyer went to his office. He had not returned by midnight, although council members continued to wait for him in chambers. Although Beavers said no final vote would be taken, people who visited Sawyer returned with differing reports.

Spectators in the chamber held dollar bills above their heads and shouted "No deal. No deal."

The delay is seen as a boost for Evans, Washington's city council floor leader. It also raises serious questions about Sawyer's resolve to continue.

"All he had to do was walk in here and he would have been mayor in five minutes," said Alderman Roman Pucinski. "But I'm not sure the momentum will be there for him Friday."

He complained the city is being run by "mob rule" and being made to look like a a "banana republic."

Sawyer allies had requested and secured a special council session the day after Washington's funeral. After Washington's sudden death last week, Sawyer had put together a formidable coalition of Democratic Party regulars, black and white, during six days of clandestine meetings, public rallies, threats, maneuvers and even a midnight exchange of signed pledges of support in a dark parking lot.

Sawyer went into the early evening meeting claiming the support of 28 council members, more than the necessary majority of the 50-member council.

But tonight Sawyer and his allies became worried about the racial composition of his support. "Gene Sawyer can't be elected totally by white aldermen. He has to have the support of the black community," Beavers said.

Evans began the day with the support of about a dozen council members, and his only hope was to press for a delay. He picked up the support of several black council members who had been supporting Sawyer, after a Washington memorial service attended by 11,000 Monday night turned into a pep rally for Evans. He picked up additional support tonight as public support grew for his candidacy.

Alderman Bobby Rush, an Evans supporter, said at the memorial service that voting for Sawyer would be "like spitting on Harold Washington's grave."

"We should treat our black enemies the same way we treat the Ku Klux Klan," said Vernon Jarrett, a newspaper columnist and adviser to Washington.

Several speakers, including Democratic presidential candidate Jesse L. Jackson, urged Washington supporters to demonstrate at City Hall.

The maneuvering over Washington's successor began minutes after the mayor died of a heart attack last Wednesday. Evans, Washington's chief political spokesman, began as the odds-on favorite. All he had to do, it seemed, was hold together the 26 hard-core Washington supporters on the council.

But the strength of the coalition was illusory. The 26 represented an uneasy alliance of blacks, Hispanics and white liberals. Many of the blacks in the coalition, including Sawyer, were less than enthusiastic about Washington's reform agenda.

"Some of the black councilmen had been abused pretty badly by Washington. His staff isolated him. They couldn't get through to the mayor when they had problems," Pucinski said. "This was their chance to straighten things out."

Sawyer got his big boost over the weekend when white council members, after clandestine meetings, decided not to unite behind one of their own. In a crucial move, Alderman Richard Mell, who had been a mayoral candidate, delivered signed pledges of support from white members to a Sawyer ally in a North Side parking lot at 12:15 a.m. Monday.

Sawyer, 53, and Evans, 44, are similar in many ways. Both have southern roots. Evans was born in Hot Springs, Ark., and moved to Chicago as a teen-ager; Sawyer grew up in Greensborough, Ala., and moved here three years after college.

Both came from Democratic organization politics and have spent much of their adult lives on the city payroll. Evans went to work for the city legal department immediately after law school; one of Sawyer's first jobs here was as a chemist in the city water department.

But their bases of support are widely divergent. Evans, Washington's city council floor leader, was embraced by political and civil rights groups that had backed Washington as the strongest and most articulate candidate to carry on Washington's "reform" agenda.

Sawyer, whose middle-class South Side ward regularly produces the largest Democratic majorities in the city, drew his main support from old machine regulars.

The whites in this group were some of Washington's biggest opponents. Sawyer rode to Washington's funeral with Alderman Edward Burke, a leader of the bloc of 29 whites who battled against Washington during the "Council Wars" of his first term.

Sawyer, first elected in 1971, has served as an alderman longer than any other black. He runs his 6th Ward Democratic organization the old-fashioned way. He buys shoes for needy voters and occasionally pays their heating bills. Each Thanksgiving he hosts a dinner for 1,000 elderly people.