MOSCOW, Sept 12, 2007 — President Vladimir V. Putin accepted the resignation of Russia’s prime minister on Wednesday and nominated Viktor A. Zubkov, the low profile head of a financial crimes agency, to assume the post.

The moves signaled the beginning of an internal Kremlin rearrangement ahead of Parliamentary elections in the late fall and Mr. Putin’s expected transfer of presidential power next year. But an aura of mystery filled the day. Kremlin watchers and diplomats said they were perplexed by what Mr. Putin’s pick meant for the pressing question of who might succeed him if he steps down next spring at the end of his second term, as he has said repeatedly he would, and as is required under Russia’s constitution.

The instant rise to prominence of the unheralded Mr. Zubkov, a confidante of Mr. Putin, added a new element of suspense for the two Russian officials long regarded as the principal presidential contenders: First Deputy Prime Minister Sergei B. Ivanov and First Deputy Prime Minister Dmitry A. Medvedev.

Neither of the men was elevated by the shifts, nor was one pushed aside.

In principle Russia holds elections by popular vote, including those for president. In practice, though, the ballots are carefully managed by the Kremlin, and are won handily by sanctioned candidates who receive support from the government and state-controlled news media.