In a traditional presidential New Year's message televised tonight at midnight, Mr. Putin assured Russians that the handover of power would take place without a hitch, and without any abrupt changes in Russia's foreign or domestic policies.

''I am drawing your attention to the fact that there will be no power vacuum, even for a moment,'' he said in the midnight broadcast. ''There has been no vacuum, nor will there be one. I want to warn that any attempt to exceed the limits of the law, and Russia's Constitution, will be decisively crushed.

''Freedom of speech, freedom of conscience, freedom of the press, the right to private property -- all these basic principles of a civilized society will be reliably protected by the state.''

The president's unexpected decision, made on the eve of what is traditionally Russia's most festive holiday, was seen by some experts as a brilliant if manipulative stroke by a politician who once again showed that he has few peers on Russia's turbulent political scene. Several politicians, interviewed during the day on television, said they understood that the president had made the decision himself, perhaps as late as Thursday night, when he inexplicably failed to make an appearance at a New Year's Kremlin reception.

''It was a very strong move by the president,'' said Vladimir Ryzhkov, a member of Parliament. ''He did it at the most appropriate time, when popular support for Putin is at a high. Early elections will hamper all of the opposition. This will increase Putin's chance of being elected many times over.''

Although Mr. Yeltsin has been in and out of the hospital since 1996, when he underwent heart surgery after his re-election to a second term, his resignation today was widely attributed to political rather than health reasons. There was no suggestion from any quarter that he was forced out of office against his will.

''It is a brilliant decision, extremely precise and profound, and apart from anything else, very brave,'' said Anatoly B. Chubais, a top Kremlin adviser and the architect of Russia's privatization program. ''I think that this decision was very difficult for Boris Nikolayevich as a person.''