Braz | 06.04.2010 10:43
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 they had tried to meet with organisers of the Mayday protests, but the organisers had been totally unco-operative.