Mayor Michael R. Bloomberg, who has expanded the breadth of his personal philanthropy during each of his four years as mayor, has showered another $20 million on 400 New York City arts and social services groups through a gift to the Carnegie Corporation of New York.

The donation, announced yesterday, represents the largest of four gifts the mayor has made through the Carnegie Corporation, which has been his vehicle for giving $55 million to local charities, ostensibly anonymously. The donations will aid a vast array of local organizations across the city as the mayor seeks re-election this year.

While Mr. Bloomberg's record of philanthropy predates his mayoralty, community and cultural groups and social-service organizations in New York City are as likely to be beneficiaries these days as medical centers in Baltimore or professorships at Harvard.

Carnegie Foundation officials and an aide to the mayor confirmed, on the condition of anonymity, that Mr. Bloomberg had made the donations. That Mr. Bloomberg is the source of the Carnegie contributions has long been an open secret and cannot help but benefit the mayor politically. The donations doled out through Carnegie range from $10,000 to $100,000 per group.

"For the fourth consecutive year, through the generosity of an anonymous donor, Carnegie Corporation of New York is making grants to small- and medium-sized New York City arts and social service institutions in all five boroughs that contribute to the culture, health, welfare and vibrancy of the city," the corporation announced yesterday.

Vartan Gregorian, Carnegie's president, said that recipients of the 406 grants were determined by the corporation and "reflect the donor's wishes to reach out to a mixture of neighborhood organizations that touch the artistic life of the city and help to meet the myriad social needs of its citizens."

In an interview, Dr. Gregorian, said that he had known the donor for many years and that "his philosophy and priorities in the cultural and social realm correspond to mine."

"In all honesty, I have not exchanged two words with him about this in the past four years," Dr. Gregorian said, adding: "I have sent him copies of all the thank you letters I have gotten - letters that said, 'Thank you for this manna,' or 'We were ready to close our chamber orchestra' - just to show that anonymity does not mean one should lack gratification."

His previous annual contributions to Carnegie have ranged from $5 million to the $15 million he gave last year.

The latest beneficiaries include such wide-ranging organizations as Amethyst House, a Staten Island halfway house for women recovering from substance abuse; Alianza Dominicana, a Dominican advocacy and social services group in Washington Heights; and at least five community groups in Harlem. And many are based outside Manhattan. At least 14 groups have the word Brooklyn in them, for instance. A complete list is available at carnegie.org/sub/news/anon2005.html.

While the grants through Carnegie represent just a portion of the mayor's overall philanthropy - last year he gave a total of about $140 million in donations to more than 800 institutions and groups - they are often vital to local groups that are struggling with fund-raising and, sometimes, have also struggled with budget cuts imposed by the Bloomberg administration. And the donations cannot but help earn Mr. Bloomberg good will with the community leaders and board members connected to such organizations.

"As in past years," Dr. Gregorian said, "the corporation has not charged overhead in administering the grants so that the total gift has been distributed to institutions, thus fulfilling our obligation to the donor's intent."

Others among the latest beneficiaries are Agudath Israel, a Jewish group that runs various social programs; the Classical Theater of Harlem; the Crown Heights Youth Collective; the Louis Armstrong House, a museum in Queens; and the Puerto Rican Traveling Theater.

One group that received a donation this year is Aspira of New York City, a service organization for Puerto Rican youths that Fernando Ferrer, one of Mr. Bloomberg's Democratic challengers, was once heavily involved in.