In an event as much group therapy as campaign rally, Mayor Gavin Newsom opened his re-election headquarters Sunday with an apology.

"What I did was wrong. And for the rest of my life I'm going to have to deal with that," Newsom told a room packed with hundreds of supporters.

"Any of you who've ever made a mistake in your life know exactly what I'm talking about," Newsom said.

Newsom, 39, announced Monday he would enter counseling for alcohol use, just days after admitting to an affair with his former campaign manager's wife.

The mayor said he especially wanted to apologize to "someone I care deeply about," Alex Tourk, 39, who resigned as Newsom's campaign manager after confronting Newsom about the affair.

At the same time, Newsom showed no sign of backing down from the race to get re-elected in November, stirring a huge cheer when he said, "I am not going away."

Among the politicians on hand to provide what amounted to a support group for the mayor were former U.S. Rep. and State Sen. John Burton, who reportedly counseled Newsom about seeking treatment for his drinking. After battling a drug and alcohol addiction, Burton went on to become president of the California Senate.

"Gavin's got a lot of work to do both professionally as the mayor (and) personally," Burton told the crowd.

Newsom began attending treatment sessions this week at the Delancey Street Foundation, a program that does not rely on the 12-step philosophy of Alcoholics Anonymous but works to address the underlying reasons for excessive drinking.

Newsom's political rivals have questioned whether the mayor's personal problems have left him capable of doing his job. And reports have surfaced that some major donors could be abandoning the mayor out of disapproval for his actions.

In an e-mail to the San Jose Mercury News, well-known Silicon Valley investor and Newsom contributor Ron Conway wrote he would no longer back the mayor "without huge character improvement from Gavin."

Still, reactions on San Francisco streets to Newsom's confessions have so far seemed mostly muted in a city famously tolerant of both sex and substance abuse.

At a packed town hall meeting Saturday in the Bayview District, one of San Francisco's poorest neighborhoods, residents took the mayor to task over violent crime and affordable housing, not his drinking or his tryst.

And in socially liberal California, according to some political observers, any aspirations Newsom may have for statewide office, including the governorship, would not likely be hindered by his current difficulties.

"If he can stay sober, if he can keep his private life more discreet, and if he can stay focused on his work, then he can realistically hope to run," Jack Pitney, a political science professor at Claremont McKenna College, said in a phone interview Sunday.

Meanwhile, Supervisor Bevan Dufty, a Newsom ally, sounded a decidedly San Francisco note in a city preparing to celebrate the 40th anniversary of the Summer of Love this year.

"Being positive, being loving, being a great San Franciscan is all each of us has to do," he told Newsom supporters, "and we're going to do great in November."