Mark Rowley, Britain's national policing spokesman for counter-terrorism, said the police had made 218 arrests so far this year while dozens of vulnerable people were also being referred to de-radicalisation programmes.
"The volume, range and pace of counter-terrorism activity has undergone a step-change," Rowley said in a statement, in the latest indication of serious concern among British authorities about the threat of an attack.
Britain has been considered a target for Islamist militants ever since the Sept. 11, 2001, al Qaeda attacks on the United States, and 52 people were killed when four young Britons carried out suicide bombings in London in 2005.
In August, Britain raised its threat level to the second-highest classification of "severe", meaning an attack was considered highly likely, for the first time since mid-2011.
Prime Minister David Cameron has warned that Islamic State (IS) militants fighting in Syria and Iraq posed the country's greatest ever security risk and has promised further measures to counter a threat posed by returning veterans. Some 500 Britons are already thought to be fighting there.
Rowley's is the latest in a string of warnings from security officials and senior politicians.
Last week, he issued a notice to police officers around the country to be on alert to possible risks to their safety, while London Mayor Boris Johnson has said spies are monitoring thousands of suspects.
Just this month, detectives have arrested 14 people in operations, and one very senior officer speaking on condition of anonymity has told Reuters some suspected plotters had got close to carrying out an attack.
DEADLY ATTACKS
"We ... are running exceptionally high numbers of counter-terrorism investigations, the likes of which we have not seen for several years," Rowley said, adding that they were thwarting several attack plots a year.
"These plots are of varied sophistication, from individuals planning to carry out spontaneous yet deadly attacks, to more complex conspiracies. Almost all seemingly are either directed by or inspired by terrorism overseas."
He said a specialist police unit was removing every week more than 1,000 pieces of illegal Internet content such as videos of beheadings, with the vast bulk related to Iraq and Syria.
"Extremist groups are using social media in highly sophisticated ways in order to recruit or persuade individuals towards their violent and warped ideologies," he said.
Together with other agencies, police were carrying out about 100 "Syria-related preventative activities", with more than half leading to individuals being referred to de-radicalisation programmes, he said. Meanwhile, families have reported 66 missing relatives who they feared had travelled to Syria.
Some Islamic groups have criticised the authorities' response to the threat from militants and say it often provokes anger and resentment among Britain's 2.7 million Muslims by demonising them.
They also say the West's foreign policy plays a key role. They and other experts have warned that as such Britain is losing its fight against radicalisation.
