Russia's opposition on Thursday defiantly vowed to stage a mass protest in Moscow at the weekend contesting the results of elections, despite warnings from the authorities to scale down the event.

Around 1,000 people have been arrested in three days of demonstrations protesting what they say was mass fraud in the parliamentary polls.

The election was won by Vladimir Putin's ruling party, though with a sharply reduced majority and less than half the vote.

Concerns over the conduct of the elections have intensified, with the last president of the USSR Mikhail Gorbachev saying they should be re-run and US Secretary of State Hillary Clinton complaining the polls were not fair.

The elections were seen as a litmus test of Putin's popularity ahead of his planned return to the presidency in March 2012 polls.

The opposition says the results for ruling party United Russia would have been far worse in free polls.

Over 20,000 people have pledged on a Facebook page called 'for honest elections' to attend the protest on Saturday afternoon on Revolution Square, just metres from the Kremlin walls.

While the rally is officially sanctioned by the authorities, the permission is for a maximum of 300 people, raising the prospect that it will be broken up by anti-riot police if greater numbers show up.

"They have said 300. If more come then we will bring the organisers to responsibility," Moscow's deputy mayor Alexander Gorbenko told Moscow Echo radio, suggesting also that a different location could be found.

But ex-cabinet minister and leading opposition figure Boris Nemtsov said the meeting had to go ahead. "The authorities are trying to intimidate their own people and doing everything that the meeting does not take place."

The Facebook page emphasises that the protest will be peaceful and tells participants to bring with them balloons and flowers and pin white ribbons to their coats.

But on late Wednesday, the municipal water company Mosvodokanal announced that urgent underground water works needed to take place beneath the square, repairs that would severely impede any demonstration.

The first demonstration on Monday after Sunday's polls took the authorities by surprise. Since then police in anti-riot gear, interior ministry troops and armoured police trucks have become a visible presence in the city centre.

Some 300 people were arrested in the first night of protests and over 550 people in the second protest a night later. Dozens were also detained after a smaller attempt at a protest on Wednesday evening.

Leading opposition figures including the blogger Alexei Navalny have already been handed 15-day jail terms.

The Kommersant daily said it was unclear how many of those detained were still being held but it appeared that dozens of similar sentances had been handed out.

"The scale of arrests has not been in any way justified," said Amnesty International. "We fear that the Russian police are simply quashing opposition protest, no matter how peaceful."

Neither Putin nor incumbent President Dmitry Medvedev have made any comment on the protests. Putin, who became prime minister in 2008 after serving two Kremlin terms, on Wednesday silently filed his application to stand in presidential elections.

Medvedev, who was visiting the Czech Republic Thursday, held a meeting with his security council of the interior ministry and the FSB security service. But there was no indication if the opposition rallies were discussed.

The United States on Wednesday criticised the police treatment of the protesters. They had expressed concern ''about the treatment of all those being arrested who were exercising their rights to peaceful protest," State Department spokesman Mark Toner said.

There has been virtually no coverage of the opposition protests on Russian state television, but they have been discussed and promoted exhaustively on the Internet, whose use has exploded in Russia in recent years.