The Guardian reports on complaints that the police were using agents provacateurs at the G20 protests.

Liberal Democrat Tom Brake says he saw what he believed to be two plain-clothes police officers go through a police cordon after presenting their ID cards.

Brake, who along with hundreds of others was corralled behind police lines near Bank tube station in the City of London on the day of the protests, says he was informed by people in the crowd that the men had been seen to throw bottles at the police and had encouraged others to do the same shortly before they passed through the cordon.

See:
http://www.guardian.co.uk/politics/2009/may/10/g20-policing-agent-provacateurs.