LONDON Police said on Monday they feared a "summer of rage" with mass protests over the economic crisis that could mar Prime Minister Gordon Brown's G20 summit in London in April.
Anti-globalisation protesters, environmental activists and anti-war demonstrators are all planning events before and during the meeting of world leaders.
Superintendent David Hartshorn, head of London's public order policing branch, said banks, multinational companies and other financial institutions could all be targeted.
"We've got G20 coming and I think that is being advertised on some of the (Web) sites as the highlight of what they see as a 'summer of rage,'" he told the Guardian newspaper.
A number of European countries have seen unrest in the wake of the economic crisis while previous G8 meetings have also acted as catalysts for violence.
Police arrested 91 people at a summit in Scotland in 2005 while anti-capitalist protests in London's financial heartland in 1999 and 2000 saw marchers clash with riot police, causing millions of pounds of damage.
Scotland Yard said there was no intelligence yet that there would be a "wave of potentially violent mass protests" or a return to riots that broke out in a number of cities during economic downturns of the 1980s.
"What police do believe is that there has been a re-emergence of some known activists who may attempt to once again become part of the protest scene in London," a police spokesman said.
"Plus with environmental and economic issues affecting more people, this may broaden the appeal of demonstration."
Protests are due to kick off on March 28 with a "Put People First" march through London coordinated by unions, charities and anti-poverty campaigns.
Environmental campaigners are planning to set up a camp in the capital's financial district on April 1 when the G20 leaders arrive, while other groups are planning action for what they have dubbed "Financial Fools Day."
"At the London summit, the G20 ministers are trying to get away with the biggest April Fools trick of all time," said the "G-20 Meltdown" website.
"Their tax-dodging, bonus-guzzling, pension-pinching, unregulated free market world is in meltdown, and those fools think we're going to bail them out. They've gotta be joking!"
Other groups are also planning rallies to protest at Israeli action in Gaza and ongoing military action in Afghanistan.
"There are certain protest groups who have always attempted to target their demonstrations against things deemed by them to be part of the state establishment -- and this includes financial institutions," the police spokesman said.
(Editing by Steve Addison)
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