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.
Documents suggest Security Services were trying to recruit Woolwich suspect Michael Adebolajo and had also approached members of his family.
Police will today question one of the men suspected of murdering Drummer Lee Rigby in Woolwich, a week after the soldier's death.
After a three day search, I found the home of the grandmother of Michael Adebowale, one of the suspected killers of Drummer Lee Rigby.