Six people have been held over a possible terror plot involving Islamist extremists with potential targets in the UK, it is understood.
The threat was not thought to be imminent, but the arrests were part of a pre-planned intelligence-led operation by the Metropolitan Police counter-terror command along with armed officers.
Five men and a woman were arrested on suspicion of the commission, preparation or instigation of acts of terrorism and are being held at a south-east London police station, police said.
A 24-year-old man was Tasered during his arrest in east London, but did not require hospital treatment, the force spokesman added.
A 29-year-old man was arrested in the street in west London, a 21-year-old man and a 30-year-old woman were held at separate residential premises in west London, and two other men, aged 18 and 26, were arrested with the 24-year-old in east London.
Some of those held are understood to be British nationals.
"Searches under the Terrorism Act 2000 are being carried out at eight residential premises in east, west and north London and one business premises in east London," the spokesman said.
"Public safety remains our overriding concern."
The arrests were not linked to the Olympics, police said.