People across Russia are voting in a presidential election that is likely to return Vladimir Putin to the Kremlin for a record third term, following a four-year stint as prime minister.

Mr. Putin, who served as Russian president from 2000 to 2008, faces four challengers in the vote, which has been marred by accusations of widespread violations.

The independent watchdog agency Golos says it has received more than 2,000 reports of irregularities, including so-called “carousel voting,” a practice in which busloads of voters are taken from one polling place to another to cast ballots.

Some 200,000 volunteer poll-watchers have been deployed across the country to monitor the voting. This is in addition to another 600,000 Internet users who have registered to monitor web cameras installed in most of Russia's nearly 100,000 polling stations.

A pre-election opinion poll predicts the former Russian leader will win at least 62 percent of the vote — enough to avoid a runoff.

While casting his vote Sunday in Moscow, Mr. Putin said he is counting on a good turnout for Sunday's vote.

“I am of course counting on a good turnout, this really is important. We all know well that presidential elections are an exceptionally important political occurrence.”

Opposition leaders decried the election as a sham.

“These elections are not free and therefore they are forgeries, the authority which so-called elected, is not legitimate. That's why we'll have protests tomorrow, and we will not recognize the president as the legitimate president, president of all Russians.”

Despite the concerns, opposition leaders expect that election officials will declare a Putin victory shortly after the polls close. They are organizing mass protests for Monday in Moscow and Russia's other big cities.

Mr. Putin faces four challengers, most of whom are familiar faces in Russian politics. Communist Party head Gennady Zyuganov is expected to place second in the vote, while nationalist Vladimir Zhirinovsky and billionaire Mikhail Prokhorov, a newcomer, are expected to battle for third place. Former Putin ally Sergei Mironov placed last in the 2004 presidential election and is expected to do so again.