In his statement outside Downing Street, David Cameron said:
"What happened yesterday in Woolwich has sickened us all. On our televisions last night and in our newspapers this morning we have all seen images that are deeply shocking.
"The people who did this were trying to divide us. They should know something like this will only bring us together and make us stronger."
More top news
A van bearing the faces of some of Britain's most wanted fugitives will be driven around parts of Spain popular with expats in a new push.
A list of commemorative events as the UK marks a decade to the day 52 were killed and hundreds injured in London terror attacks.
The veteran US comedian made the admission in a 2005 civil case, newly-unsealed court documents show.