Street, 56, who shared power with Rendell at City Hall before resigning 11 months ago to seek the mayoralty, won on the strength of massive support among black voters and a modest but decisive level of backing from blue-collar whites.

Katz conceded about 1:20 this morning. "I spoke to John Street a few minutes ago and expressed my congratulations," the Republican candidate told cheering supporters at the Park Hyatt at the Bellevue. "John ran a very, very strong campaign."

Street spent the night huddled with supporters at the Warwick Hotel, following returns which showed him leading Katz as returns from the city's 1,681 polling places were counted.

Enough pro-Street voting divisions had filed returns by midnight to make the crowd at the Warwick upbeat and excited. The ballroom was so packed that security guards began to stop people from entering.

Mayor Rendell told interviewers earlier in the day that he expected that Street, who served in City Council for 19 years, would win by several percentage points.

Several hundred thousand peopled scurried along the rain-slicked streets of Philadelphia yesterday to cast their ballots in a down-to-the-wire race that had given the city its greatest political excitement in many years.

Katz, 49, an entrepreneur and government consultant who has specialized in sports-arena financing, was seeking to become the first GOP mayor since Barney Samuel left office in 1952.

John P. McDermott, 48, the Constitutional Party candidate, was not expected to get more than a fraction of the votes. But in a close race, it was possible for him to tilt the outcome. Street partisans were hoping McDermott could get 2 percent of the vote, calculating that, as an abortion foe from the Northeast, he would deprive Katz of some votes.

To deal with the problems of municipal services, declining neighborhoods and population loss, the new mayor will have to forge a working relationship with a 17-member City Council, also elected yesterday.

In addition, early returns showed voters overwhelmingly in favor of a change to the City Charter that would give the mayor power to appoint all nine members of the school board within a few months of taking office in January. That would make him more accountable for how the schools operate.

Polls last week suggested that the mayor's race was razor close, but the dynamics of any political contest can shift in the last few days as undecided voters make up their minds.

The result was also expected to be influenced by the level of voter turnout - by whether Street was able to achieve the high turnout among loyalist Democrats, particularly African Americans, that he needed to win.

The voter-registration rolls contain the names of 988,005 people, including 736,692 Democrats and 191,742 Republicans. But experienced political observers believe that many of these people were among the 150,000 who have left the city in the last decade.

The number of citizens able to vote was probably between 700,000 and 750,000.

Katz, as the nominee of a party that is outnumbered by a 4-1 ratio in the city, needed massive numbers of voters to cross party lines.

Katz, a former candidate for mayor and governor, entered the race with a proven ability to raise money. A former Democrat, he shied away from the Republican label. He won the support of the city's leading gay and lesbian organizations and took moderate positions that gave liberals and minorities no reason to fear him.

In Philadelphia, the mayoral contest is rarely close. Race clearly has played a role here. Though Street and Katz ran exemplary campaigns in which neither appealed to voters on the basis of skin color, both knew that a majority of blacks would probably vote for Street and that a majority of whites, regardless of party, would vote for Katz.

The last close race was in 1987, when Democrat W. Wilson Goode was elected to a second term by 17,000 votes over former Mayor Frank L. Rizzo, on the ballot as a Republican. That, too, was a biracial election.

The closest race of the last half-century was the 1967 contest pitting Democratic incumbent James H.J. Tate against Republican Arlen Specter, now a U.S. senator. That was decided by 11,000 votes.

Specter said last week he knew the mental torture Street and Katz were going through as they awaited the vote.

"It's very disconcerting to know that every minute voting machines are clicking off your fate," he said. "The tension and the anticipation are very strong."

Both Katz and Street spent many weeks - and millions of dollars in campaign advertising - appealing to the swing voters, especially white Democrats, who could have gone either way.

In the final weeks of the campaign, Street had sought to capitalize on the advantage that party label figured to give him. He cast the race not as Street versus Katz, but as Democrat versus Republican.

He ran a TV ad featuring Rendell, his most important supporter. He ran an ad featuring District Attorney Lynne M. Abraham and three other prominent local Democrats. He brought in Dennis Archer, the Democratic mayor of Detroit, and Anthony Williams, the Democratic mayor of Washington.

His final card was his hoped-for trump. On Friday, four days before the election, President Clinton campaigned for Street at a rally on the campus of La Salle University.

In the rain yesterday, Street continued to drum up enthusiasm in sections of the city where he expected strong support.

In the late afternoon, Street, with an entourage of campaign aides, went to the Broad and Olney subway station in the city's Ogontz section, where he spent the final four hours of his campaign greeting hundreds of homeward-bound commuters.

Street's "Polaroid posse" - aides with Polaroid cameras - took photographs of the candidate posing with commuters in the station concourse as they got off the Broad Street Subway trains.

For hours, a stream of people lined up - at times, as many as 20 people at a time - to stand next to the candidate in the glare of a camera flash. Campaign aides said they had taken 3,600 photos of Street with voters during the day.

As hordes of commuters climbed the steps from the subway platforms, Street aides steered them toward the smiling candidate, who stood in front of a pillar dressed in a blue suit, white shirt and striped tie.

Ogontz and surrounding neighborhoods were viewed as a Street stronghold.

The subway concourse at times took on the air of a political rally. Some commuters chanted "John Street, John Street, John Street" as they moved through the station.

A passing woman grabbed a bullhorn from one of Street's campaign workers and shouted: "John Street for mayor."

Brian Knight, 26, a computer student who was heading home with his 4-year-old daughter, Breona, stopped to get his photograph taken with Street.

"He's for the people," said Knight, "and he has experience."

Street arrived at the subway stop at 4 p.m. By 6, he looked tired. But he predicted victory.

"I've always felt good about my chances," he said. "I've always believed in the end Democrats would vote Democratic."

Asked how turnout was, Street gave a one-word answer: "High."

Voters, he said, had turned out well despite periods of pouring rain. "People don't care about the rain."

An aide, Ray Jones, said: "We've heard nothing but good things. The turnout is twice what it was in the primary in North Philadelphia. Rain has not affected us one bit."

But even in the closing hours of the day, Street's workers were urging people to vote Democratic.

A Street field director, Manwell Glenn, shouted at the crowds: "We've got to stop the Republicans. Don't forget to vote."

Unlike Street, Katz kept a low public profile yesterday. He voted early in the morning at the Lutheran Theological Seminary at Germantown Avenue and Allens Lane in West Mount Airy.

Katz planned to spend most of the afternoon at home with family - including his daughter Lauren, who flew in from Tufts University, in Boston. Though the candidate kept in close touch with his daughter by cellular phone throughout the campaign, he had not seen her since dropping her off at college earlier this fall.

Though Street toured the city, Katz had no such plans, campaign director Bob Barnett said.

"There's nothing you can do at that point," Barnett said.

At Katz's campaign headquarters on the 28th floor of the Lewis Building at 15th and Locust Streets, the phone rang constantly. Some callers wanted to know where they could vote. Others wanted to know if it was too late to vote by absentee ballot.

One 76-year-old caller said that after she voted for Katz this morning, the sanitation department did not pick up her trash.

Downstairs in the lobby, wet volunteers lined up for assignments to make calls from the Bellevue or to hand out fliers and knock on doors to spread the Katz message.

Later, at the Bellevue, the guests included Happy Fernandez, a defeated candidate in the Democratic mayoral primary.

She acknowledged she had not been to many Republican parties. But the former City Council member, who endorsed Katz, said she was comfortable with her decision.

"I'm proud of what I did. It's the right thing," she said. "Both men focused on the issues and ran a clean campaign." They differed on leadership and personal style, she said. "Sam was willing and ready to cross party lines and racial lines. He was out in the neighborhoods talking to voters. That's what a mayor must do - be there."

At Street headquarters, Democratic Party committee members and trade-union workers joined first-time political volunteers in munching potato chips while waiting for Street to appear.

The action was on TV, where anchors reported the latest tallies out of the city computers.

Ken Snyder, Street's spokesman, said: "We're meeting projected vote totals in almost every area, and we're running up huge margins in North Central, Northwest and West Philadelphia. We're taking 95 percent in these divisions."

It was almost midnight. Still, neither Street nor Katz was ready to claim victory.