Britain's security services are monitoring thousands of terrorism suspects in London and are involved in operations on a daily basis, the capital's mayor Boris Johnson said in an interview published on Saturday.
"In London we're very, very vigilant and very, very concerned," mayor Johnson told the Daily Telegraph newspaper.
In August, Britain raised its international threat level to the second-highest classification of "severe", meaning an attack was considered highly likely, with Prime Minister David Cameron warning that Islamic State (IS) militants fighting in Syria and Iraq posed the country's greatest ever security risk.
About 500 Britons are believed to have joined the fighting in the region and the authorities have long argued that radicalised, battle-hardened veterans would pose a grave threat on their return.
However, Johnson suggested the danger was more widespread.
"Every day - as you saw recently, we had to raise the threat level - every day the security services are involved in thousands of operations," Johnson told the paper.
"There are probably in the low thousands of people that we are monitoring in London."
The risk posed by homegrown radicals was most vividly illustrated by the deadly London suicide bombings in July 2005 by four young British Islamists.
Last year two British Muslim converts hacked to death a soldier in broad daylight on a London street, and on Tuesday five young men were arrested by police as part of what was described as an operation against Islamist-related terrorism.
Johnson's comments come the day after Britain's most senior counter-terrorism officer Mark Rowley warned police around the country to be alert to possible risks to their safety.
"Measures are being put in place to increase the vigilance of officers and staff," Rowley said.