Five men, including four Iranian nationals, were arrested at locations across England on suspicion of preparation of a terrorist act after a suspected plot "to target a single premises" was uncovered, the Metropolitan Police said.
The force confirmed a further three Iranian men were arrested on the same day - Saturday - as part of a separate operation.
British Home Secretary Yvette Cooper said the arrests were sparked by "serious events that demonstrate the ongoing requirement to adapt our response to national security threats".
Those arrested include a 29-year-old man in the Swindon area, a 46-year-old man in west London, a 29-year-old man in the Stockport area, a 40-year-old man in the Rochdale area and a fifth man in the Manchester area. They remain in custody.
Another three Iranian men were arrested in London. Two men, aged 39 and 44, were arrested at separate addresses in northwest London and the third - a 55-year-old man - was held in west London.
They remain in custody and searches continue at the three addresses.
Commander Dominic Murphy, head of the Met's counter-terrorism command, called it "a fast-moving investigation".
"The investigation is still in its early stages, and we are exploring various lines of inquiry to establish any potential motivation as well as to identify whether there may be any further risk to the public linked to this matter," he said.
"We understand the public may be concerned and, as always, I would ask them to remain vigilant and if they see or hear anything that concerns them, then to contact us."
14°C