Russian law enforcement is bracing for widespread protests as Russians cast their ballots in a presidential election that is considered a sure victory for Vladimir Putin.

Waves of anti-Putin protests have rocked Moscow and other cities in recent months. Opponents were angered by the former president's decision to run for the country's top job yet again after a four-year hiatus mandated by Russia's laws against more than two consecutive presidential terms.

There's also concern over electoral fraud, an issue that has repeatedly filled Russia's streets with protesters in the last few months.

December parliamentary elections that favoured Putin's United Russia party were rigged, opponents say, and that's only stoked the fear of fraud during this presidential vote.

Rather than boost confidence in Sunday's vote, the use of video and web cameras in combination with an estimated 200,000 monitors at more than 90,000 polling stations has merely drawn suspicion, said CTV's London correspondent Ben O'Hara-Byrne.

"Vladimir Putin's opponents say that if he's trying to make this election transparent, it's because he's already found a way to win," O'Hara-Byrne told CTV News Channel from Moscow Saturday.

Despite protesters' calls for Putin to hold the parliamentary vote again, O'Hara-Byrne said he has brushed those requests aside.

If Putin wins Sunday, which is the expected outcome, it's unlikely the disputed parliamentary elections will be held again.

"There will be allegations, no doubt, that this election was rigged. There's already protests planned by the opposition for Monday as well as by pro-Putin groups," O'Hara-Byrne said, explaining that Putin is hoping to avoid the vote from going to a potentially embarrassing second ballot.

Instead, he's aiming to claim the 50 per cent of the popular vote necessary for a win on the first round.

"People in Russia are not going to recognize Putin's victory in the first round," Alexei Navalny, an outspoken and charismatic member of the opposition, declared earlier this week.

Police in Moscow are preparing for an onslaught of protesters regardless of the outcome.

Russia's Interior Ministry has called in 6,000 police reinforcements to Moscow, according to state news agency ITAR-Tass.

Rather than a vote for a new president, O'Hara-Byrne said, Sunday's election is shaping up as more of a referendum on Putin.

"I think there will be a lot of interest in this election and there's certainly a lot more at stake than people originally thought there might be back as early as six months ago," O'Hara-Byrne said.

"This is Putin's race to win or lose . . . and a lot of the people voting for other candidates are also primarily voting against Vladimir Putin."

According to the most recent survey by the independent Levada Centre polling agency, Putin is on track to win the election with around two-thirds of the vote against four challengers.

None of the other candidates have been able to marshal a serious challenge to Putin.

The Communist Party candidate, Gennady Zyuganov, gets support of about 15 per cent, according to Levada, which claimed accuracy within 3.4 percentage points.

The others - nationalist firebrand Vladimir Zhirinovsky, Sergei Mironov of A Just Russia and billionaire Mikhail Prokhorov - were all in single digits.

Experts say the measures designed to ensure a more open and transparent vote may not work.

"Cameras cannot capture all the details of the voting process, in particular during counting," the election observation mission of the Organization for Security and Cooperation in Europe said in a report on election preparations.

Observers from the agency, as well as thousands of Russian volunteers, will be monitoring events at polling stations.

But during December's parliamentary vote, observers from Golos, a non-governmental agency, said they were threatened and ordered to leave polling stations.

"The Russian government has done the right thing by allowing unprecedented public protests and proposing some reforms," Hugh Williamson of the international watchdog Human Rights Watch said in a statement. But "despite the positive developments, the climate for civil society is as hostile as it ever was."