London is on the highest security alert in its history, with 1,500 extra officers to be drafted on to the streets of the British capital by the weekend.
"London is at risk because it is one of the world's major capital cities, although there has been no specific threat," Metropolitan Police Commissioner Mr John Stevens said today.
Britain has been the most vociferous supporter of the United States in President Bush's "war against terrorism," sparking fears that London could become the next prime target.
Police are also particularly eager to protect British Muslims from any retaliation after last week's devastating attacks in Washington and New York that killed more than 6,000 people.
High-visibility patrols will guard mosques because community leaders fear trouble after Friday prayers.
Since the attack on the United Sates, police in Britain have reported a number of assaults, including one on a young Muslim woman and another that left an Afghan taxi-driver paralysed.
Schools and mosques have been targeted with threats against pupils and graffiti attacks.
A radical British-based Muslim leader has denied giving his followers a "licence to kill" Pakistan leader Gen Pervez Musharraf for backing the United States over the New York and Washington attacks.
There was a storm of outrage in Britain following reports that Sheikh Omar Bakri Mohammed, who leads the Al-Muhajiroun Islamic group, had issued a fatwa or religious ruling that called for the death of Gen Musharraf.
Radical Muslims have been angered by Gen Musharraf's decision to back the United States in its efforts to track down its main suspect, Saudi-born militant Osama bin Laden.