Detectives are questioning a group of suspected Islamist terrorists amid fears the men were plotting to launch an attack in Britain.
ITV News has obtained dramatic CCTV footage of one of a series of arrests across London, in which counter-terror police are seen tackling a 28-year-old suspect on the upmarket Westbourne Grove in west London.
George Paul witnessed the arrest of one of the suspects in Westborne Grove and told ITV News he saw "three policemen holding down one individual - I don't know what ethnic origin he was but he was shouting.
The Metropolitan Police said searches are being carried out at six addresses after four people were arrested on suspicion of terror offences.
In a statement, Scotland Yard said: "At approximately 19.10 today (Sunday, 13 October) four men were arrested on suspicion of the commission, preparation or instigation of acts of terrorism under the Terrorism Act 2000.
"They have all been taken to a south London police station. Two men were arrested in Mansell Street, E1. One man was arrested at a premises in Westbourne Grove W2. The fourth man was arrested in Peckham Street, SE15.
"Searches under the Terrorism Act 2000 are being carried out at six addresses in London. Officers from the MPS Firearms Unit were involved in these arrests. Public safety remains our overriding concern."