Thirteen men were arrested under Britain's Anti-Terrorism Act following a series of raids across the country, the police said.
Police swooped on the men, aged in their 20s and 30s, on Tuesday in a well planned operation in north-west London, Hertfordshire, Bedfordshire and Lancashire.
"The operation is part of continuing and extensive inquiries by police and the security service into alleged international terrorism," the Metropolitan Police said.
The men are being held on suspicion of being engaged in the commission, preparation and instigation of acts of terrorism under the UK Terrorism Act 2000.
The raids were led by the Metropolitan Police's Anti-Terrorism Branch and supported by officers from
Hertfordshire, Bedfordshire and Lancashire.
A Metropolitan Police spokesman said the men were taken to a central London police station for interview by anti-terrorism officers.
According to the police, officers are executing several search warrants in London, Bushey, Luton and Blackburn.
The arrests come as British Prime Minister Tony Blair is under pressure to spell out the level of the terrorism threat in the country.
The United States is already on heightened alert, following the seizure of computer files from al-Qaeda suspects captured in Pakistan apparently mentioning potential targets for attack both in the US and Britain.
"We are maintaining a state of heightened readiness in the UK," the British Home Office said.
