Thousands of police will cram the streets as George W Bush visits Britain.
And so will many tens of thousands of people very angry about Bush's bombing of Afghanistan and Iraq, his military build-up, and his blocking of the Kyoto agreement on the environment.
Clashes are likely. In fact, if it weren't that the Blair government is so boneheadedly arrogant, you would have to think that they had chosen to fete Bush so ostentatiously only in order to provoke clashes.
On May Day 2001 the police also made a large, demonstrative mobilisation. They said there was a danger of "violent demonstrators". In fact there was no significant violence from demonstrators. But the police cordoned off thousands of people, holding them in the streets for many hours to stop them demonstrating.
On 31 March 1990 one of the biggest demonstrations in London before the recent anti-war ones, up to 200,000 marching against the Tories' poll tax, ended in riots. 113 people were injured and over 500 arrested, some of whom later got long prison sentences.
Even the demonstration organisers were originally shaken, and said they would "name names" of disruptive demonstrators to the police. Soon it became clear that the violence had been started by police provocation.
Remember that if there are big clashes on 20 November.