MOSCOW, January 21 (RIA Novosti) - The Russian Central Election Commission Monday registered Russian First Deputy Prime Minister Dmitry Medvedev as a presidential candidate.

In mid-December, Putin publicly backed Medvedev as his successor. Medvedev, 42, board chairman of Russian energy giant Gazprom, was earlier nominated by the ruling United Russia party as a presidential candidate. Putin agreed to be prime minister should Medvedev win the March 2 election.

United Russia said it is convinced Medvedev will ensure continuity for Putin's policies.

Earlier, the leaders of the Communist Party and the Liberal Democratic Party, Gennady Zyuganov and Vladimir Zhirinovsky, were registered as candidates.

Ex-Russian prime minister Mikhail Kasyanov has some problems with the voter signature sheets, the Central Election Commission's secretary said Monday.

"There are questions about the voter signature sheets collected in some regions on behalf of self-nominated candidate Mikhail Kasyanov," Nikolai Konkin said, adding that the problems were related to insufficient information about those who collected the signatures, which is a serious violation and could result in all signatures in support of his candidature being declared invalid.

Konkin said the leader of the Democratic Party, Andrei Bogdanov, was also experiencing some problems, but did not elaborate.

A final decision on the registration of Kasyanov and Bogdanov should be made no later than January 27.