Mayor Bloomberg is giving $125 million of his personal fortune, a good chunk of what he would have to spend to run a competitive presidential campaign, to reduce smoking worldwide.

The donation will go to track smoking across the globe and to push for the same kind of smoking bans and cigarette tax hikes that the mayor has implemented in New York since taking office in 2002.

“This century, a billion people will die from smoking. It is one of the world’s biggest killers and it has sadly been overlooked by the philanthropic community,” Mr. Bloomberg said yesterday.

The move is in keeping with Mr. Bloomberg’s record of philanthropy, but is unusual because he announced it publicly rather than writing an anonymous check, as he often does.

Although his decision to go public with the gift could be interpreted as an attempt to raise his profile in preparation for a 2008 presidential run, Mr. Bloomberg reiterated that he plans to devote himself to philanthropy when he leaves City Hall. He pointed out that he has purchased a building on the Upper East Side to house his foundation.

The donation was praised by politicians and by industry groups such as the Campaign for Tobacco Free Kids as an unprecedented commitment.

“What it says is that we have a mayor who is devoted to the public good,” Mayor Koch said. “I along with millions of New Yorkers appreciate his having extended the cigarette prohibitions here in New York City. I think it’s marvelous that he’s taken it a whole new level. If he can have an impact on saving lives worldwide we should all be very proud of him.”

Others said that using the money to push for higher cigarette taxes and smoking bans was another example of Mr. Bloomberg imposing a father-knows-best style at the expense of personal choice.

“At least this time it’s his own money,” a senior fellow at the Manhattan Institute, Walter Olson, a critic of the smoking ban, said. “Unfortunately, part of what he’s trying to do is to get all these new laws acted on the theory that Bloomberg-knows-best and that second-guesses decisions of other grown people.”

Mr. Olson said he is fine with Mr. Bloomberg spending money on antismoking campaigns and programs to help people quit, but that forcing governmental restrictions goes too far.

“Having made so much money in a system of liberty it’s too bad that he feels obliged to spend it in a way that reduces other people’s liberty,” Mr. Olson added.

The mayor has touted the city’s smoking ban — which was masterminded by his health commissioner, Dr. Thomas Frieden — as a smashing success.

He said the city has 200,000 fewer smokers in the city now than four years ago because of the crusade against tobacco.

As a billionaire whose net worth has been estimated at $5.1 billion by Forbes magazine, Mr. Bloomberg has a long history of giving and an even longer history of favoring public health initiatives, even when it’s meant distancing himself from his fellow Republicans.

Last year, the mayor gave $100 million to his alma mater, Johns Hopkins, where the school of public health his bears his name. A portion of that money went to support stem-cell research and pediatric medicine.

In June, he gave an “anonymous” $30 million donation to Carnegie Corp., which distributed the money to dozens of groups around the city. The Chronicle of Philanthropy ranked Mr. Bloomberg no. 7 on its list of top charitable donors last year after he gave $144 million to 850 groups.

The latest donation will go to existing organizations that have not yet been publicly identified. Part of the money will fund the development of a “rigorous system to monitor the status of global tobacco use.”

That data-driven approach is a signature trait for Mr. Bloomberg. He invokes the same statistical philosophy when making decisions about city programs.

A professor of public administration at Columbia University, William Eimicke, said he’s never been bothered by smoke in bars and, if the money were his, he would probably choose another cause. But it’s the mayor’s money, he said, and the issue is clearly important to him. Another political spectator joked that Mr. Bloomberg was now moving to raise his international profile rather than just his national profile in an attempt to become secretary-general of the United Nations, rather than president.

Mr. Bloomberg said yesterday that he stopped smoking 25 to 30 years ago and while he thinks the smell of a cigar is pleasurable, evidence shows that inhaling smoke first- or second-hand is deadly.

“We all have an obligation to help our fellow man and each of us can do that in a different way at a different level depending on our situation,” Mr. Bloomberg said.