LONDON—The number of police deployed on the streets of London on Tuesday will be boosted from 6,000 to 16,000 following a third night of rioting in the British capital, Prime Minister David Cameron said.

“The Metropolitan Police Commissioner has said that compared with the 6,000 on the streets last night, there will be some 16,000 on the streets tonight,” Cameron said after a meeting of the country’s national emergency committee.