From the BBC website: "Similar Mayday anti-capitalist events have been marred by violence. Metropolitan Police said some arrests had been made overnight.
'Additional heat'
Ch Insp Mark Payne said police knew who some of the main protagonists were.
"As part of the preparation we have made some arrests overnight to deal with some people who might be placing additional heat on the demonstration.
"We look very closely at who we think it is going to cause trouble and we have people on the ground who will be able to indentify them to local officers," he said.
It is not clear how many arrests were made or on what grounds they were made.
Police said it tried to meet with organisers of the London Mayday demo but they had not been co-operative.
 http://meltdown.uk.net