Britain is bracing for a week-long series of protests in the run-up to next week's G20 summit in London, which will see fortress-like security for world leaders including US President Barack Obama.
Demonstrations are set to kick off as early as Saturday, going on through the summit next Thursday, while police have already warned of others who may use "ambush" tactics to try and catch them out.
The threat of terrorism is also taken seriously -- and while there is no specific information about an attack, police are on high alert with the national terror threat level at "severe", meaning an attack is "highly likely".
Some 2,500 police will be deployed in London, along with more to protect and escort leaders from the summit venue in east London's docklands to receptions at Prime Minister Gordon Brown's Downing Street offices or Buckingham Palace.
All holidays for officers in London have been cancelled for the summit, businesses have been advised not to hold any unnecessary meetings, and police have been called in from five other forces to bolster available resources.
"This is a challenging week," said Commander Bob Broadhurst, who is leading the 7.2 million pound (10.5 million dollar, 7.8 million euro) operation. "It is not often you get 20 world leaders plus."
Police began searching the ExCel Centre, where the summit will be based, and other key sites on Tuesday, while a marine unit will patrol the conference site from attempts to break in by boat from the Thames river alongside.
Throughout, demonstrations with causes ranging from action on climate change to demands for an end to Western involvement in Iraq and Afghanistan, will be hoping to draw world leaders' attention.
On Saturday, a rally led by Put People First, an umbrella group of trade unions, anti-poverty campaigners and green groups, will be held in central London's Hyde Park, urging G20 leaders to protect jobs and public services.
On April 1, protesters will attempt to disrupt the capital's financial district, known as the City, through a series of anti-globalisation and global warming demonstrations, dubbed the "Four Horsemen of the Apocalypse".
On the same day -- when leaders including Obama are expected to be already in town -- the Stop The War Coalition will lead its own rally calling for international troops to be withdrawn from Iraq and Afghanistan.
Early the next morning, several protest groups are aiming to gather outside hotels near the summit venue where the G20 leaders will be staying in a bid to create sufficient amounts of noise to disturb their sleep.
"We have to be flexible and mobile," Broadhurst said. "One of my concerns with public order policing is if we police the last problem we had we will be caught out."
"It will be an exciting couple of days to say the least."