(Recasts, adds denial, protest by opposition parties)

ST PETERSBURG, Russia, March 5 (Reuters) - A Russian opposition activist has gone on hunger strike after being detained on the eve of protests against Dmitry Medvedev's election as president, his party said on Wednesday.

Maxim Reznik, head of the St Peterburg branch of the liberal Yabloko party, was detained just hours after polls closed in the election in which Medvedev won more than 70 percent of the vote.

Police said Reznik was detained after he attacked another man on the street and later assaulted a policeman who rushed to the scene, charges that Reznik's party denied.

"He refused to take food once he was detained," Alexander Shurshev, a senior Yabloko activist, told Reuters. "On Tuesday he filed an official statement on starting a hunger strike."

On Tuesday, a court in Russia's second city extended his detention in a local jail by two months for the alleged attack on a policeman.

A party spokesman said Reznik's detention was part of an attempt to undermine Yabloko before two opposition rallies against the election of Medvedev, President Vladimir Putin's protege.

The St Petersburg meeting, which drew 2,000 people, was largely peaceful but in Moscow, where the rally was banned, riot police arrested scores of activists, Reuters reporters at the scene said.

Western observers criticised Sunday's election, in which Medvedev won more than 72 percent of the ballot, as not fully democratic. Opposition candidates were either barred from running or refused to take part in protest.

Reznik, 33, graduated in 1996 with distinction from the history department of St Petersburg University, where Medvedev had studied a decade earlier. (Reporting by Denis Pinchuk; Writing by Dmitry Solovyov and Guy Faulconbridge; Editing by Richard Balmforth)

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