Polling stations across Russia have opened as the presidential election gets under way.

Sunday's vote sees Vladimir Putin hoping to become president again after four years as prime minister.

Putin was Russia's president from 2000 to 2008, but was barred by the constitution from standing for a third consecutive term.

He faces four challengers, three of whom he has defeated in previous elections.

The election is being held against a backdrop of popular discontent, sparked by allegations of widespread fraud during December's parliamentary elections in favour of Putin's United Russia party.

Polls will be open from 8am to 8pm in each time zone, with the first opening in the Far East of the country at 20:00 GMT on Saturday, and the last in the western Kaliningrad region closing at 17:00 GMT on Sunday.

The interior ministry is bringing 6,000 police reinforcements to Moscow from the regions, according to Russian media reports.

Putin's main challenger is considered to be Communist Gennady Zyuganov, who is running for a fourth time.

The other candidates are ultra-nationalist Vladimir Zhirinovsky, tycoon Mikhail Prokhorov, who is standing as an independent, and former upper house speaker Sergey Mironov, from the centre-left A Just Russia party.

If Putin fails to achieve more than 50% of the vote he will face his nearest rival in a run-off.

However, the liberal opposition behind some of the recent protests is not represented.

The so-called white-ribbon movement has attracted more than 50,000 people in recent demonstrations in Moscow and other major cities, after widespread allegations of vote-rigging during elections for the State Duma. Similar numbers attended pro-Putin and pro-Communist rallies.

Putin has responded by announcing a programme to install webcams in each of the country's 90,000 polling stations, but critics have questioned their effectiveness.

"Cameras cannot capture all the details of the voting process, in particular during counting," said a report by the Organisation for Security and Co-operation in Europe (OSCE), quoted by the Associated Press.

A joint mission by the OSCE and the Parliamentary Assembly of the Council of Europe comprising 250 observers is monitoring the elections.

Meanwhile tens of thousands of Russians have volunteered as election observers and been trained to recognise and report violations.