Prime Minister Vladimir Putin acknowledged on Wednesday he might not win in a March presidential election in the first round, and said Russia could face political instability as a result.

Putin, who is facing the biggest opposition protests of his 12-year rule, remains the strongest candidate and looks set to win a six-year presidential term.

But public opinion polls suggest he could fall short of 50 percent of the votes and face a runoff, which would be an embarrassing blow that could undermine his authority in a new stint in the Kremlin.

There is nothing terrible about it (second round), if needed, I am ready to campaign in the second round, Putin told a group of volunteer election monitors.

I also understand that a second round implies an extended period of certain infighting, meaning a certain destabilisation of our political situation, Putin said.

Opinion survey results released on January 25 by independent polling agency Levada showed 37 percent of respondents would vote for Putin, 17 percent were undecided and another 9 percent uncertain whether they would vote at all.

A runoff, would likely pit Putin, 59, against veteran Communist Party leader Gennady Zyuganov, who came second in the Levada poll with 8 percent.

Putin's strategists have launched a low-key campaign appealing to Putin's core supporters in the provinces and brushing aside a barrage of criticism from the opposition concentrated mainly in large cities.

There is a danger that my supporters may not come (to the polls), thinking that I will win anyway, that they have nothing to do there, that we will do something to ensure my victory. We will do nothing like that, Putin said.

CONVINCING RESULTS

After alleged fraud in a December parliamentary election triggered mass protests, Putin said he was keen to ensure that the presidential election is fair and ordered the installation of web cameras at polling stations across Russia.

I want the country to understand that the election will be fair, open and the result will be objective. In order to make the result a convincing one, everyone who wants to vote for me or for someone else should not wait for the runoff, he said.

Another Putin rival, billionaire Mikhail Prokhorov, backed by 4 percent in the Levada poll, told a news conference that a runoff would be a blessing for the country. It means that political competition and a democratic system would be born.

After election authorities barred opposition politician Grigory Yavlinsky from standing, some analysts have speculated that Prokhorov played a part in a Kremlin plan to split the opposition and lend the vote an air of competitive legitimacy.

In a bold display of disapproval at Putin's plan to return to the presidency, his opponents erected a huge banner reading Putin, go away on Wednesday on a rooftop facing the Kremlin. Police later removed the banner.

The opposition plans to bring thousands of people to the streets of Moscow and other large cities on Saturday, trying to keep up the momentum gained after mass protests in December.

Putin's supporters plan to stage rival rallies, accusing the liberal opposition of being paid by foreign powers to destabilise Russia. Putin's ally Deputy Prime Minister Dmitry Rogozin called on Russian nationalists to back Putin.

Those who rock the political boat are aiming at Putin. You know why? Because Putin is the symbol of policy independent of Washington, Rogozin wrote in an article published in pro-Kremlin daily Izvestia.

(Reporting by Gleb Bryanski; Editing by Rosalind Russell)