Britain's police chief said Sunday that several suspected terrorists linked to Al Qaeda are under surveillance and more arrests are expected.
Metropolitan Police Commissioner Sir John Stevens said Al Qaeda leader Usama bin Laden and his associates have acted "very cleverly" in tapping into existing networks in Britain.
Stevens said on Sky News television that "a large number of people" were under surveillance.
"There's still a number of people who still have yet to be arrested, but as I said we are working very, very closely with the security services," Stevens said. "We know that there's certain links with Al Qaeda."
Police have arrested a number of people under Britain's anti-terrorism legislation in recent weeks.
Four North African men were charged Monday with chemical weapons and terrorism offenses in relation to the discovery of the deadly poison ricin in London on Jan. 5.
Ricin, derived from the castor bean plant, is one of the world's deadliest toxins and has been linked in the past to the Al Qaeda terror network and Iraq.
Three North African men also were arrested Wednesday in a second raid in the northern city of Manchester that police said was linked to the ricin discovery.
One of those men, Kamel Bourgass, 27, appeared in court Friday, charged with the murder of a police officer who was stabbed during the raid.
Stevens said police did not know whether weapons of mass destruction were falling into the hands of terrorists in Britain from rogue states.
"We know these people are quite prepared to give their lives, they are extremely ruthless and they are prepared to use weapons which perhaps people who have been involved in domestic terrorism have not been prepared to use, so, therefore, there is a need for us to up our game and we are doing that," he said.
The stabbing death of Detective Constable Stephen Oake, 40, in the Manchester sweep raised questions about how well British police are equipped to deal with terror suspects. The arrested men had not been restrained and some of the police were unarmed and wore no body armor.
British police usually do not carry guns, though in cities they sometimes wear protective clothing.
Stevens said police and security services had their best information ever on the network of terrorists and stressed they were "on top of" the situation.
He said there was not yet a need to arm all police officers on the street.