Oval incident involving 173 innocent people who might have been about to commit crime

I've been offline (and will be for the next week or so) due to the house move, but heard more than the usual wailing of sirens and saw a lot of blue lights near to Oval tube station last night. There was clearly something more going on than cricket.

This morning we have the following round up:

1. The Metro reports that a "mob of 180 youths" who apparently only "planned" to unleash chaos at Notting Hill were stopped yesterday.

2. The BBC report that "About 180 youths were stopped and seven arrested outside Oval as police prevented a large group from going to the carnival". Out of the 150 or so that went to the police station, only "seven were formally arrested". Even more outrageous, "Police said they believed the group, which included many teenagers, were on their way to the carnival to create disorder and criminal incidents." And of course, it's definite that they might have been about to commit crime. After all, "Some members of the group had a history of being involved in disorder at the carnival, police said." The drama all took place in Harleyford Road, which was closed off.

3. Bathsheba at Stockwell News asked what was going on, and received reports of helicopters and police presence.

Is it just me, or does it appear somewhat wrong that a large number of {innocent} people were stopped by police, a huge number of {innocent} people were then detained, and only seven were arrested? I mean, the BBC report rather implies that these were people that only might have been about to commit crime. Can you really refuse to allow somebody to go to the Notting Hill Carnival on the basis that they might commit crime? And from all of the "intelligence" gained, why was it only possible to arrest seven people?

Of course, we're not living in a police state...