From keeping the WikiLeaks site alive to helping British anti-austerity protesters outmaneuver riot police, Twitter is entering the Western political mainstream as a powerful tool for dissent.

The website where anyone can post 140-character messages with links to multimedia content was praised by the United States last year when Iranian protesters used it to organize and spread news after a disputed presidential election, until Tehran blocked the site as part of a wider clampdown.

Now, as campaigns around both WikiLeaks and British student protests demonstrate, it is showing its effectiveness in new ways as a thorn in the side of established bodies and authority.

Twitter has been at the heart of WikiLeaks' fight to survive. It pointed followers toward mirror sites in Europe after Wikileaks.com was taken down as part of a wider campaign against it following the leaking of U.S. diplomatic cables.

As cyberattackers shut down the MasterCard corporate website on Wednesday in apparent retaliation for its blocking of payments to WikiLeaks, Twitter was the vehicle for a group styling itself Anonymous to claim responsibility.

In recent weeks, British students protesting against government plans for a near-trebling of university tuition fees have mobilized the power of the new medium in a battle of wits with police that has also erupted into physical skirmishes.

POLICE CHASE

Last week, police in body armor and fluorescent jackets found themselves literally sprinting to catch up with student demonstrators. Students who had found themselves hemmed in by riot police the previous week used Twitter and smartphones to tip each other off about the location of police lines, and dodged them by breaking into small groups.

The thing about Twitter is the speed with which it allows information to be disseminated, said Carina O'Reilly, European security analyst at IHS Jane's. So many of that generation are plugged into it almost continuously. It allows action without centralized leadership. It's one of the reasons the police have been so far behind the curve in the protests so far.

Undergraduate Jessica Riches, 20, primary operator of The twitter feed @UCLoccupation -- representing student protesters occupying a key part of University College London -- said it proved an incredible way to gain publicity and raise funds.

To start with, we weren't sending that many tweets and I didn't think many people would be watching, Riches -- who normally runs a campus fashion blog at in her spare time -- told Reuters.

But then we saw how many people were retweeting and it really took off from there. It's been incredible. Without Twitter, I don't think we would have been successful.

INFLUENTIAL AUDIENCE

The tactic helped inspire a string of other university occupations and grabbed the attention of politicians and broadcasters -- both increasingly using social media sites to monitor public opinion, particularly among the young.

Politicians who have made Twitter a key part of their communications strategy can find their walls peppered with requests for support or solidarity -- or simply with abuse.

New British campaign #ukuncut -- which has grown out of the student protests but has broader aims of pushing back austerity measures in general -- is using Twitter to organize flash mobs at businesses it accuses of tax avoidance. It intends to keep expanding, regardless of what happens over tuition fees.

The student protests and WikiLeaks are in some ways two sides of the same thing -- the way the Internet is changing the world, said David Lea, Western Europe analyst at Control Risks.

It's not that the technology is new this year -- it isn't. It's that enough people are using it for it to make a difference and that politicians are taking it seriously.

Most analysts believe the student protest will not be enough to stop tuition fee reform passing into law in a vote in parliament on Thursday. But the campaign has won some concessions and piled pressure on the government's junior coalition partner the Liberal Democrats -- once popular with students, now hated by many for going back on a pledge not to raise fees.

Many now expect Twitter to be at the heart of wider anti-austerity campaigns in Britain next year as proposed spending cuts gather pace. Disability activists are already using it to trade complaints and protest plans ahead of expected benefit cuts.

If we lose the vote on Thursday, I think some people will drift away, said Riches. But I think some will get involved in wider protests. The market is there -- politically engaged young people who want to act but also a much wider range of older people, mothers, trade unionists.

(Additional reporting by Georgina Prodhan, editing by Mark Trevelyan)

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