* Russia to hold parliamentary election next month
* Liberal opposition says vote is being rigged
* Kremlin says Russian elections are free and fair

Russian opposition leader Mikhail Kasyanov said on Thursday parliamentary elections next month were being rigged against his party, meaning it would have to win up to three times more votes than legally necessary to get into parliament.

Starved of air time, villified by Kremlin-backed media, and physically attacked on the stump, Kasyanov and his allies in the People's Freedom party or PARNAS face an uphill struggle to break into the 450-seat lower house of parliament on Sept. 18.

Despite an economic crisis, the main pro-Kremlin United Russia party is expected to comfortably win the elections, which are seen as a dry run for Vladimir Putin's presidential re-election campaign in 2018.

The crisis means United Russia's margin of victory may be slimmer than recent years however, giving PARNAS, which currently has no seats in parliament, a glimmer of hope.

If it did manage to break through, Kasyanov, who served as prime minister under President Putin from 2000-2004 and who earlier this year said he feared for his life, said his first move would be to try to impeach Putin for taking Russia ‘to the edge of the abyss.’

‘We see that mass falsifications are already being prepared,’ Kasyanov, 58, chairman of PARNAS, told a Moscow news conference on Thursday. ‘They (falsifications) happened in recent years, in 2011 and 2012, and they will happen this year.’

The Kremlin rejects suggestions Russian elections are rigged and says Putin and United Russia are genuinely popular.

The Central Election Commission appointed a new chairwoman known for her human rights work this year and Moscow is allowing the OSCE to observe next month's elections.

But Kasyanov pointed to amendments to Russia's election laws which he said made it harder to monitor whether voting was fair and cited election experts as saying the authorities were able to routinely falsify up to 5 percent of the vote meaning his party would need to outperform.

‘To be able to form a fraction in the Duma (parliament) we will have to win 10-15 percent of all votes,’ he said, explaining that the authorities would disregard the real result for PARNAS and ‘write in’ a new lower result that would keep its support around the 5 percent mark.

The necessary threshold to get into parliament is 5 percent but if PARNAS wanted to get over the line with say 6 percent, Kasyanov said it would need to win 15 percent to take account of the likely scale of falsification.

The most recent opinion poll conducted by the independent Levada Center in July suggested PARNAS might manage 7 percent.

‘We need to win a lot more votes than can be falsified,’ said Kasyanov, who told Reuters in a February interview he feared for his life after fellow activist Boris Nemtsov was gunned down last year.

Kasyanov, who was in February shown in the cross-hairs of a sniper's rifle in a video posted on the internet by the pro-Russian boss of Chechnya, was last week attacked by pro-Kremlin activists on the stump.

He suffered a setback in April when a state-backed TV channel broadcast footage of him in bed with an activist discussing opposition infighting. Kasyanov said the secret services had put a hidden camera in his flat.

Several opposition allies said he should step down after that episode saying that, fair or not, it had created an image problem for the party. Kasyanov refused.

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