Police are urging the public to be vigilant
Scotland Yard warned of a dual threat from dissident Irish republicans and international terrorists, as officers launched a pre-Christmas anti-terrorism campaign in London.
Officers were continuing to question a 35-year-old man on Thursday, arrested in Derbyshire by police investigating bomb attacks by the Real IRA.
A farm and three houses in Derbyshire and Nottinghamshire are being searched.
Londoners were warned they needed to remain vigilant and to provide information if the city was to remain safe.
A newspaper and radio campaign will concentrate on the threat from the Real IRA, who have been linked to a series of bombings on the mainland in recent months.
Suspicious packages
Detectives initially had 48 hours to question him but will be able to apply for extensions of up to one week under the Terrorism Act.
David Veness, Assistant Commissioner of Specialist Operations for the Metropolitan Police said the threat of new attacks could not be ignored.
He said: "This year, above all others in recent memory, it is apparent to everyone that London faces a range of terrorist threats."
Mr Veness reminded people about the importance of reporting suspicious packages to the police.
"The Metropolitan Police is doing everything possible to combat the threat of terrorism but it is only with the help and support of the public that we can reduce the harm which it causes," he added.
Mr Fry added: "Terrorists need somewhere to live, somewhere to store their equipment, and they need vehicles to move around in.
"If you sell cars or let property and you have noticed something suspicious please contact us."
Since June 2000 there have been a series of terror attacks in London.