Prime Minister Vladimir Putin's party is struggling to hang onto its majority in Russia's parliamentary election, results have shown.

United Russia held a two-thirds majority in the outgoing State Duma, which allowed it to change the constitution unchallenged.

But with ballots counted in 75% of precincts, United Russia's share of the vote had dropped to about 50%. This was in line with an exit poll conducted by the VTsIOM polling agency which had United Russia on 48.5% and another by FOM which placed it on 46%.

The Communist Party appeared to benefit from the protest vote, with exit polls and the early returns predicting it would get nearly 20%, up from less than 12% four years ago.

About 60% of Russia's 110 million registered voters cast ballots, down from a 64% turnout four years ago.

Rival parties and election monitors suggested 50% support for United Russia would be inflated. They alleged ballot-stuffing and other significant violations at the polls. Only seven parties were allowed to field candidates for parliament this year, while the most vocal opposition groups were barred.

Mr Putin hoped widespread backing for United Russia in Sunday's election would be a sign of popular support for his return to the presidency in a vote now three months away.

Despite the sobering setback, he was still expected to have little trouble reclaiming the position he previously held from 2000 to 2008.

Mr Putin has systematically destroyed any potential challengers and most Russians do not see any credible alternatives, despite growing dissatisfaction with his strongman style. Grumbling over pervasive official corruption and the gap between ordinary people and the super-rich has become widespread.

Putting a positive spin on the disappointing returns, Mr Putin said: "We can ensure the stable development of the country with this result.