15:42 The autonomous bloc is in a pen, but people are allowed out in one and twos.
15:30 The March has now finished at Trafalger Square where there is now a rally. The police had briefly stopped the autonomous bloc from entering the square. There was a brief scuffle as the police pushed in to seize a small bike sound system. Others gathered around shouting "solidarity" and "let them in".
14:40 Demo took 40 minutes to pass Bush House with about 4000 people. International, Iraq, Iran, Sri Lanka, Nepal, Turkey. The three largest blocks on the demo are the RMT, TNG (unions) and the autonomous bloc.
13:20 The colourful trade union march is setting off - complete with autonomous bloc at the back with a banner saying "autonomous", red and black flags and a sound system.
12:47 The TU demo assembles with about 2000 people at the moment - and very colourful. The Autonomous bloc has is just behind the CPB.
11:46 About 100 riders set off on the critical mass with chilling sounds from a sound system at the front.
11:15 About 50 people are at the CM, but more are arriving. It's well chilled and people are eating. Police and protestors alike are looking at the book stalls under the bridge. Two officers are collecting info. And the sun is finally managing to warm the place up.
This is not quite right. People were surrounded but they were allowed to leave in ones and twos, but not together. They weren't detained as such, just surrounded and prevented from moving anywhere together.