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.
The arrests were not linked to the Olympics, police said.
The arrests come after Jonathan Evans, the director-general of the Security Service, warned last month that Britain had experienced a "credible terrorist attack plot about once a year since 9/11".
"In back rooms and in cars and on the streets of this country there is no shortage of individuals talking about wanting to mount terrorist attacks here," he added. "The threat is real and remains with us today."
The threat to the UK from international terrorism is currently rated substantial - the third highest of five levels. The rating is set by the Joint Terrorism Analysis Centre (Jtac), based at MI5's headquarters at Thames House in central London, but is independent of the service.