World leaders have their photo taken at the start of the G20 summit Press Association

POLICE snatch squads were on standby today to arrest G20 rioters responsible for violent attacks on officers and buildings in the Square Mile.

The Met said anyone involved in violence should expect a "knock on the door" after specialist officers began trawling through hours of CCTV footage to identify ringleaders of yesterday's trouble, which saw 87 arrests.

Thirty people - including three police officers - were injured and one man died of a suspected heart attack. Police officers who tried to resuscitate the protester were pelted with bottles.

Commander Simon O'Brien said: "Today's focus remains on ensuring that both the summit and lawful protest takes place, but if we see a target from yesterday's disorder then we will arrest them.

"Yesterday's protesters were mostly peaceful but clearly there were a couple of hundred who were hellbent on causing trouble - throwing things, punching officers and trying to stoke up the crowd.

"Most of the arrests came last night when many peaceful protesters had gone home, but some remained and continued to face up to officers and about 60 are still in custody."

Images of anarchists filmed by high-powered cameras from the Met's helicopter and officers on the ground armed with camcorders are being checked with intelligence records in an effort to identify them.

The warning came as a ring of steel was thrown around the ExCel Centre for today's meeting of world leaders, chaired by the Prime Minister.

"Today's focus remains on ensuring that both the summit and lawful protest takes place, but if we see a target from yesterday's disorder then we will arrest them."

An estimated 4,700 police officers were on duty including snipers, river boat patrols and scores of armed diplomatic protection officers as presidents, prime ministers and delegates from across the world began arriving in the Docklands.

Officers armed with sub-machine guns manned roadblocks around the east London site and everyone arriving at the centre went through airport-style security checks.

DLR stations near ExCel including Royal Victoria, Prince Regent and Custom House were also closed in an effort to stop protesters from getting close to the summit.

Across London protests continued with at least nine demonstrations taking place.

Around 40 protesters converged on the Stock Exchange in Paternoster Square for a giant game of Monopoly. Police made two arrests.

Two hundred people also took part in a march from Tooley Street near London Bridge to ExCel.

A huge clean-up operation was underway in the City today to remove graffiti and anti-capitalism slogans daubed on walls.

The windows of a Royal Bank of Scotland branch in Threadneedle Street which was stormed by anarchists were also being boarded up.

Stuart Fraser, of the City of London Corporation, said police had "handled a tricky job very well".