Five men and one woman were reportedly rounded up in a series of raids that included use of smoke grenades and a stun gun. One of the locations was a home close to Olympic Park.
Scotland Yard said the detainees were planning terror attacks, but the attacks had no link to the Summer Olympic games, which begin July 27.
Scotland Yard confirmed it had previously employed one of the detained men as a community support officer, a law enforcement worker who patrols the streets but does not have powers of arrest. The department said he resigned, and had not been involved in any sensitive work.
Resident John Smallshaw said a raid took place at a residence on Abbey Road, only about a mile (1.6 kilometers) from the Olympic Park. He told The Associated Press he was awoken just after 4 a.m. by "five loud bangs in quick succession" and saw police raiding the home. He said he later witnessed "one young man taken on foot to a waiting ambulance."
Searches were being carried out at eight separate addresses in east, west and north London and at one business in the Hackney district of east London, close to the capital's financial district.
Police said all the suspects have been taken to a southeast London police station to be questioned. They can be held for questioning for 14 days before they must be charged or released.
Security officials said the arrests followed an intelligence operation to monitor the group. No attacks were considered to be imminent, and the fact that Britain's terrorism threat level has not been altered also indicates that authorities believed they had good knowledge of the extent and scope of the possible plot.
Britain's terror level is currently ranked as substantial - the third highest point on a five-point scale - and means an attack is a strong possibility. Intelligence officials say there has been an expected increase in chatter among extremist groups ahead of the games, but there are still no specific or credible threats targeting the Olympics.
The Associated Press contributed to this report.