I arrived late, there was 70 odd people* there and police lines were already moving people. After mingling for a short period police formations basically forced people into a pen (which was predicatble and I cant understand the oversight tbh) which when sealed held about 40 odd people, a few people managed to get out.
After the usual banter, and exchanges with the police they informed us we were being held under the breach of peace act indefinitely, while the BNP roamed around unmolested - and that if we wished to do so we could leave in ones and twos (although this initially included searches and photographing from what I gather). Within four hours most had left the pen but some tried to insist on being allowed to leave collectively, but this didnt happen again under the terms layed down by the police that we were a threat to the peace.
The embankment turned out to be a bit of entertainment for a few onlookers (Im not sure how sympathetic some of them were tbh) we even had the fellow from newsnight down

some werent and we got the usualy atagonistic cunts asking dumb questions, but some 'support' later turned out to be anarchos who were shouting
In terms of assessing the event I think it was a mess, and Im not inclinned to think more preparation, more resources etc would make the event any better. Im not slagging people off btw, I supported the event and dished out a share of the leaflets. I actually think showing opposition to Boris, the BNP et al is important, its just a shame it was just anarchos and their millieu. We have to discuss this openly and seriously address our lack of influence in London and wider.
* although my understanding is a few people had already begun to dissipate by this point