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One of the most influential new figures to emerge in Russian politics predicts that Vladimir Putin's grip on power will crumble within a year and a half.

Alexei Navalny became well known as an anti-corruption activist, using his internet blog to expose widespread excesses and outright theft by Russia's political elite.

But he is now becoming even more prominent as one of the informal leaders of huge protests in Moscow. Tens of thousands took to the streets in December and this month.

In a ´óÏó´«Ã½ interview - unusual because he rarely speaks in depth to Western media - he explained why he believed the government's time was running out fast.

"A lot of people inside the establishment understand that corruption cannot be the foundation of the system," he said. "You can't function like that forever, and it's not right, and sooner or later it will come tumbling down."

"I don't think that Vladimir Putin and his circle will be able to sustain the system he built for more than another 18 months."

When asked if Russia needed a peaceful revolution he turned the question round saying "the truth is that the criminal revolution was carried out by Vladimir Putin.

"He really usurped power, took power in an illegal way. Vladimir Putin staged a revolution and what we need to do now is to return power to the people."

He said he and others in the opposition were surprised when 100,000 people took to the streets in the recent protests, and warned that further fraud in the presidential elections would provoke an even bigger reaction.

"If the falsifications on the fourth of March - and we know there will be falsifications - are as obvious and irritating to people as they were in December's Duma [parliamentary] elections I think even more people will come out onto the streets."

Many liberals in Russia are distrustful of Alexei Navalny because he attended the Russia March, a nationalist demonstration with strong far-right connections. But he insisted that his aim in going there was to make sure that the issue of immigration control was not ignored by more mainstream Russian politics.

"When people say the word 'nationalism' it immediately conjures up negative associations," he admitted. "But the truth is that when we talk about so-called nationalist topics, we're talking about real problems.

"Russia is in second place for illegal immigration. My taking part in events that traditionally only nationalists attend, and not liberals or democrats, comes from the fact that I think we shouldn't leave the discussion of these very important questions to marginal politicians."

Such strategies havegiven Alexei Navalny a much wider appeal, beyond just the Moscow middle classes. He has supporters in small towns right across Russia, the common thread being an internet connection, and a sense of outrage at the levels of corruption in the country.

Almost half of Russians now have access to the internet.

"TV is completely closed to us," he explained. "All of the big traditional media outlets are under state control. That's why the internet became for us the only instrument. We can do everything for free, or almost for free, and we can reach out to quite a lot of people. So a lot of political activists like me built their strategy on the internet, because they know there is no other way."

This week a light-hearted political programme called Gosdep on MTV Russia, on which he had been invited to appear, was cancelled. Not just the one show, but the whole series.

It was Alexei Navalny who first coined a famous phrase to describe Vladimir Putin's United Russia Party: the "Party of Crooks and Thieves". His reward has been an increasing number of pro-Kremlin videos posted on YouTube that attack him.

"It's a sign that what I'm doing is working," he said. "I understood perfectly that our job was to do something so that these crooks who are stealing from our country live badly.

"Naturally they will try to make my life harder. The growing pressure from the Kremlin shows that I really am being effective and my work is a real threat to them."

He admitted that Vladimir Putin was still the country's most popular politician, but said the true support was only around 30%, and even that was achieved by state control of television, and to a lesser extent the newspapers.

"These days his popularity is based on a monopoly of the media - Putin puts all his faith in television," he said. "They destroyed all of the politicians around him, and there is this huge PR machine exclusively serving him. But even this PR machine can't cope any more."

Alexei Navalny says he is not running for election - yet - because the voting system needs to be fixed first.

"Our task now is to secure fair elections," he explained. "That is task number one. Once we have done that I am sure that great new politicians will appear as candidates for the posts of prime minister, Duma deputies or president. And I think I will be among those people and I will fight for a leadership position."