David Cameron asked for the Downing Street flags to fly at half mast following news of the death of Nelson Mandela.
Tamil solidarity campaigners are protesting outside Downing Street in central London against David Cameron's attendance at the Commonwealth Heads of Government Meeting in Sri Lanka this evening.
Leaders from Canada, India and Mauritius have stayed away from this year's summit in protest at the Sri Lankan government's human rights record.
Police in London found a cache of weapons including a 950,000 volt stun gun in the car of a man who said he was on his way to kill the Prime Minister, a court has heard.
Irfaq Naz, 34, also had hammers, kitchen knives, a machete, an unsheathed Samurai sword and masking tape in his Vauxhall Astra when he was stopped in July after going the wrong way down a north London street.
Officers later searched his home in Middlesbrough and found the Downing Street postcode and the names of other prominent political figures.
The taxi driver was detained under the Mental Health Act, and last month admitted possessing a prohibited weapon, three counts of possessing an offensive weapon, seven counts of having a bladed article and dangerous driving.
The Cobra meeting, which lasted around an hour, heard that security has been tightened at Woolwich barracks and increased at all London barracks, Downing Street said.
The meeting was attended by the Home Secretary, Defence Secretary Philip Hammond, Mayor of London Boris Johnson, Met Commissioner Sir Bernard Hogan-Howe, Met Assistant Commissioner Cressida Dick and intelligence agencies.
The meeting was told there are strong indications that it was a terrorist incident.
An armed diplomatic protection group police officer who bunked off duty to meet his lover has been jailed for nine months. Ricci Giff, whose job included guarding Downing Street and anti-terrorism work, left his patrol of London's most sensitive buildings on three occasions in late 2011
Married Giff, of Sandy, Bedfordshire, who has two young children and 15 years experience as a police officer, admitted three counts of misconduct in a public office. He was jailed for what Judge Anthony Pitts said was behaviour that could not be tolerated by an officer in such an important role.
Giff looked visibly shocked when he was given an immediate custodial sentence.
Judge Pitts told him what he did during their meetings was not important but the fact that he was unable to respond if there had been an emergency was.
The Prime Minister was briefly buffeted by the wind and snow as he left Number 10 for the House of Commons.