Four people are being questioned following a series of terror arrests on Thursday as part of an investigation into a suspected plot in the UK.
Armed police raided a house in Ilfracombe Gardens, Romford, and a flat in Frank Slater House, in Green Lane, Ilford, both in east London.
A man aged 31 and a 17-year-old boy were held at one address, along with a 27-year-old man at the other. A man aged 32 was arrested in Leicestershire.
Police said they all remain in custody.
BBC News home affairs correspondent Daniel Sandford said sources indicated the arrests were part of a potentially “significant” counter-terrorism operation and related to a suspected plot in the UK.
No shots were fired during the arrests in east London, but loud distraction grenades could be heard across the area as one raid took place at Frank Slater House.
The 27-year-old man who was held at one address was taken to hospital after he was bitten on the foot by a police dog, but his injures are not thought to be serious.
Armed police were not involved during the arrest in Leicestershire.
The teenager and the three men are all being held on suspicion of being concerned in the commission, preparation or instigation of acts of terrorism, thought to be Islamist-related.
Scotland Yard said searches were being carried out at all three properties, as well as a fourth in east London.