Five men and a woman have been arrested on suspicion of the commission, preparation or instigation of acts of terrorism, police said.
The six arrests in London were part of a pre-planned intelligence-led operation by Scotland Yard's counter terrorism command along with armed officers, but were not linked to the Olympics, police said.
A 24-year-old man was tasered during his arrest, but did not require hospital treatment, a force spokesman added.
The six were taken to a south-east London police station, the force spokesman said.
A 29-year-old man was arrested in the street in west London, a 21-year-old man and the women were arrested at a separate residential premises in west London, and three other men, aged 18, 24, and 26, were arrested in east London.
"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 added. Public safety remains our overriding concern."
The operation is understood to relate to a possible plot involving Islamist extremists with potential targets in the UK.
The threat was not thought to be imminent.
Some of those arrested are understood to be British nationals.