A man was arrested after entering the secure area of Downing Street late last night.
The man was spotted at the back of Mr Tony Blair's residence by officers at 10.36pm, challenged and then arrested. He is being questioned at a central London police station.
The intruder scaled the fence at the rear of Downing Street, in Horseguards Parade, and entered the outer secure area. He was then challenged by a police officer and arrested after a brief struggle.
The man was held on suspicion of affray and assaulting a police officer. He was found to have a knife.
The intruder lunged at the officer, but it is not thought that he wielded the knife during the struggle.
A Scotland Yard spokesman said: "We are satisfied that at no time was the prime minister at risk."