"The activities undertaken by the anti-terrorist branch specifically in relation to London and nationally with the security services has prevented in some instances a possible terrorist attack on this city," Metropolitan Police Commissioner John Stevens said.
He declined to provide details of the threatened incidents in his address to a meeting of the watchdog Metropolitan Police Authority, but said he could brief members in private.
"I want to emphasize the high state of alert this city is on at the moment," Stevens said.
"There has been a massive amount of work taking place in the anti-terrorist branch and security services for the last year and a half," he said, adding that the work still continued "in a major way."
Stevens said David Veness, head of the anti-terrorist branch of the police, had reported that "he is two and a half to three times busier now than he has ever been in his extensive experience."
The government says that since the terrorist attacks on the United States on Sept. 11, 2001 and the American-led war on Iraq, Britain is a possible target for attack.
Stevens has warned that Osama bin Laden's al Qaeda network — blamed for the Sept. 11 attacks — has a substantial presence in Britain. Because of its links to former colonies, a large portion of British immigration comes from Middle Eastern countries where terrorist groups have made inroads.
In January, traces of the deadly poison ricin were found in a London apartment. Hundreds of troops were deployed around London's main Heathrow airport in February because of fears that terrorists might use a surface-to-air missile to down a jetliner.
Police have staged simulated terror attacks in the British capital and offered advice to Britons about how to stay safe. Police forces in England and Wales are training teams to deal with a chemical, biological, radiological or nuclear attack.
London has dealt with the threat of imminent terrorist attack for no less than 60 years.
The Irish Republican Army conducted a bombing campaign in Britain in the 1950s and struck London several times in the intervening years — including a 1984 attempt on Prime Minister Margaret Thatcher and a 1993 bombing that cause $1 million in damage.
In 1980, Iranian dissidents took over their country's embassy in London. After six days, British special forces troops stormed the building, killing five terrorists.