A first sample of the signatures was declared to include 20% invalid. A second sample is being examined now according to commission officials.

The election commission already has registered Putin and three other contenders: Communist Party chief Gennady Zyuganov, ultranationalist Vladimir Zhirinovsky and socialist Just Russia leader Sergei Mironov. Since their parties are represented in the parliament, their registration is easier than for other potential candidates.

Election officials also signaled Monday they would register billionaire tycoon Mikhail Prokhorov, saying that a preliminary check of lists of signatures in his support had shown they correspond to legal norms.