Policed and peaceful: Tens of thousands of people marched on Saturday
By Dominic Casciani
Police officers spend their professional lives trying to play down the public order implications of demonstrations - it's in their interests to keep things calm.
But the enormous logistical challenges of providing the world's leaders with security, amid widespread expectation of substantial protests, has led the capital's top officers to issue exceptional warnings about this week's G20 meeting in London.
Last week, Commander Simon O'Brien, one of the senior officers involved in planning the events, said the capital was about to see an "almost unprecedented level of activity" with seven officially notified demonstrations - and potentially many more they don't know about.
It's little wonder that Scotland Yard is describing the G20 policing plan as one of the largest, most challenging and complicated public order operations it has ever devised.
Events begin on Monday with the three-day state visit to the UK by the President of Mexico.
At the same time, thousands of officials from the other G20 delegations will be arriving in London.
On Wednesday, large "direct action" events are predicted in the Square Mile (although by no means guaranteed) in the shape of the over-lapping "Fossil and Financial Fools Day" protests and the "G20 Meltdown".
At the same time the seemingly well-resourced Climate Camp group says its supporters will try to build a tent city in the middle of one of London's busiest roads to protest against the carbon trading market.
G20 LONDON SUMMIT
World leaders will meet later this week in London to discuss measures to tackle the downturn. See
our in-depth guide
to the G20 summit.
The G20 countries are Argentina, Australia, Brazil, Canada, China, France, Germany, India, Indonesia, Italy, Japan, Mexico, Russia, Saudi Arabia, South Africa, South Korea, Turkey, the United Kingdom, the US and the EU.
Meanwhile, a couple of miles away, the Stop the War coalition will march from the US Embassy to Trafalgar Square.
And then as evening approaches, there's the small matter of Wembley turning on the floodlights for England's World Cup qualifier against Ukraine.
As for Thursday, the day of the G20 itself, all police can honestly say at the moment is that the picture is "still emerging".
And did we mention that London is a city where there might be a few people trying to get to work and back?
So when officers describe this coming week as a "fluid and dynamic situation", that's something of an understatement.
The security strategy for all of this boils down to something that resembles a three-dimensional ever-changing puzzle. The Met needs to be prepared for a virtually unlimited number of scenarios.
Some 84,000 police man-hours have been allocated to the entirety of Operation Glencoe, the G20 security strategy. All police leave has been cancelled in London for Wednesday and Thursday.
Six police forces are part of the £7.5m security plan. The Metropolitan Police is naturally leading - but also calling in colleagues from the City of London and British Transport Police.
Outside of the M25 motorway, officers from Bedfordshire, Essex and Sussex will have critical roles in securing the arrival and transfer of delegations to their virtual bunkers in embassies and hotels.
Don't expect to see President Barack Obama waving from his bomb-proof limousine; he'll walk down the steps of Airforce One at Stansted Airport and board his presidential helicopter for the short hop to the West End.
Inside the capital, police will co-ordinate the movement of these entourages and create a sterile environment at the Excel centre, base for the talks in East London's Canning Town.
The security arrangements at Excel are so rigorous that Newham Council is warning some residents will find it difficult to get into their own homes.
Three Docklands Light Railway stations will close, along with the roads nearest the centre. Pedestrian access will be severely limited and residents will need to carry two forms of identification.
Police intelligence suggests the return of "some old faces" to the protest scene - although officers will not be drawn publicly on what that means.
Police want "dialogue" with groups planning demonstrations
The level of activity on the net, and its style, suggests a re-emergence of groups which share the aims and tactics of some of the anti-globalisation protesters who turned to violence in 2000 and 2001.
While the Met has tried-and-tested tactics to deal with violence, its strategy for the street occupation promised by the Climate Camp is more difficult to call.
Scotland Yard will not be drawn on how it will respond if the camp turns into something significant, other than its commanders have "flexible plans".
It admits that its overall plans are being informed by the wealth of information now being posted online by the protest organisers.
This internet activity is also helping City firms decide how to prepare. Firms spoken to by the BBC were reluctant to go on the record about any of their specific plans.
Many have turned to private security consultants, who, in turn, are keeping a watchful eye on any specific threats that emerge online. At present, City of London Police are telling firms to cancel unnecessary meetings and deliveries, beef up building security and keep a low profile.
Each year the Met deals with about 4,500 events requiring visible public order policing. They range from innocuous Boy Scout marches through to the recent angry scenes outside Israel's embassy.
Police believe that if there is a big turn-out on Wednesday and Thursday, the vast majority of people will peacefully make their voices heard.
OFFICIAL ADVICE TO CITY FIRMS
No entry without ID
Check ID outside buildings first
Minimise entry and exits
Review external smoking areas
Check CCTV equipment
Don't antagonise protesters
Source: City of London Police
Past experience suggests unorthodox and imaginative street events can be inconvenient - but also benign.
The unknowable factor is the demonstrator bent on violence.
The police will be determined to avoid the internationally embarrassing scenes of the 2001 G8 in Genoa in which one protester was killed and hundreds more injured.
So with three days to go, the police say their message to protesters is clear.
"Come forward and make contact with us so we can make sure that your [legitimate] aims are achieved," says Commander O'Brien.
"But there are groups that by their very ethos won't talk to us. The groups which enter dialogue with us, we will facilitate [their events].
"We will not tolerate anyone breaking the law, be it by attacking buildings, people or our officers.
"We are looking to police peaceful protest. We don't talk in terms of riots. If anyone wants to come to London to engage in crime or disorder, they will be met with a swift and efficient policing response."