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The snipers will be in place, the manhole covers bolted, the bins removed, the cameras trained and the motorcycle outriders primed: as world leaders converge on the capital for the G20 summit Britain’s biggest security operation is swinging into operation.

Up to 5,000 police officers drawn from more than 30 forces are preparing to confront anarchists and environmentalists while 40 armed convoys containing the world’s most powerful people and protected by 1,200 security personnel will attempt to weave their way through the theatre.

A climate camp has been planned for somewhere in the Square Mile, four marches nicknamed the Four Horsemen of the Apocalypse are due to converge on the Bank of England and the US Embassy, and small bands of protesters have declared their intent to lay an early siege to the ExCeL centre in Docklands.

The unprecedented policing operation, which is costing more than £10 million according to estimates, will focus on keeping up with the changing tactics of the protesters. The organisers are telling their people to keep on the move, stay in small groups and then, on receipt of a text message, swoop on their designated “targets”.

Commanders at the Metropolitan Police public order control room in Lambeth, South London, will be watching from all angles – street, helicopter and handheld cameras – trying to work out what is going to happen next and how to prevent it.

While the Camp for Climate Change might be peaceful – all bunting and street-theatre – other protesters are threatening violence.

Anarchist groups are resurgent on the blogosphere, chattering about storming banks, attacking City workers and reclaiming the streets. No one doubts that a hard core will be intent on clashing with the police – they will cover their faces, arm themselves with makeshift weapons and try to smash windows, statues and police heads.

British-based groups, including the Whitechapel Anarchist Group, Class War and the Wombles, held a secret meeting this week to discuss tactics for the demonstrations. They reportedly plan to split up into small groups to catch police off guard and break through officers’ lines.

Under the banner of G20 Meltdown in the City, one group said that at noon “we are going to reclaim the City, thrusting into the very belly of the beast: the Bank of England”.

The City is prepared. Many offices and shop fronts have been boarded up, meetings have been cancelled and staff are prepared to work from home or to stay behind their desks.

The police say that they are ready too. They will mount what is probably their biggest security and public order operation.

The Metropolitan Police says that 2,500 officers will be on duty in the Square Mile alone today, but sources say that number is a deliberate public understatement intended to play down the seriousness of the situation.

Six forces in and around the capital are directly involved in Operation Glencoe – the codename for the vast exercise – but more than 30 others have responded to requests from Scotland Yard for specialist help, sending 600 officers so far.

The Met is at full stretch, with the prospect of facing four marches, the climate camp and an unknown number of impromptu incidents as well as a Stop the War march in the West End. It will also provide security and protection at ExCeL and around the visiting delegations, and supply officers for an England World Cup qualifier at Wembley.

The frontline officers have been briefed on tactics – how to enclose and seal off protests – on the use of public order and antiterrorism laws and on maintaining discipline. They have also heard their senior officers tell them that they will meet any disorder with a “swift and efficient response” and one commander’s boast that his men are “up for it”.

Writing on a police internet forum yesterday, one officer was ready for trouble. He talked of “going up against the scum of our society, the immature thrill seekers and anonymous cowards who hide in large crowds with scarves pulled over their faces chanting meaningless slogans to hurl whatever is at hand at the lines of police deployed to maintain order”.

The officer signed off with a rallying cry: “So boys and girls, keep your chin straps tight, your batons ready and shields high”. Others on the forum were less gung-ho, talking of keeping safe and returning to their families uninjured. All gave the impression that they were expecting violence.

The first stand-off between police and protesters occurred yesterday afternoon at a vacant office block on the edge of the City designated as a “convergence centre” where demonstrators from outside London could spend the night. The building was surrounded by police who arrested three people as they tried to gain access.

Politicians said yesterday that police warnings of violence at today’s G20 protests were in danger of becoming a “self-fulfilling prophecy”.

Four Liberal Democrats – Baroness Williams of Crosby, Simon Hughes, Chris Huhne and David Howarth – will act as legal observers at the climate camp to prevent violence initiated by police, rather than protesters.

Mr Howarth, the MP for Cambridge, said that police and media were guilty of “talking up the violence”, adding: “The danger is that they are putting off peaceful protesters, and attracting the wrong sort.”

The organisers of the climate camp yesterday met the policeman whose job it is to try to control them. The protesters wanted reassurances that they would not be met with aggressive police tactics. Commander Bob Broadhurst wanted to hear that his men and women would not come under attack. It was a tense affair.

A programme of rolling road closures is envisaged to allow convoys to speed past while minimising disruption to city life. The task has been eased somewhat by the decision to fly President Obama from Central London to City airport before driving a short distance to ExCeL.

Nevertheless, that journey, and the President’s short hops around Whitehall and St James’s, will receive the full security cover that accompanies every Cadillac One cavalcade: police outriders, armoured Secret Service vehicles and armed air cover.

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