One of the G20 protests yesterday was the Climate Camp, who set up their tents in the middle of London's streets.
Now what happens if there is an ad hoc camp site in the middle of the road? The roads become unusable for ordinary people trying to come in and out.
So the climate campers blocked other people's free movement.
So does anyone want to have a guess at what there bitching about in relation to the police's treatment of them? The police prevented them from going in or out of the protest zone.
Their free movement was blocked.
(See also Julia and LfaT).