Around 500 British police officers raided houses in a major operation targeting people suspected of planning terrorist attacks overseas.
A spokesman for Manchester police
They arrested eight people in early morning raids, seven in Manchester, northern England, and one in Merseyside, the area around Liverpool in the north west, police said.
They later said they had arrested a ninth person and that one of the nine had been released.
"An extensive operation targeting individuals suspected of facilitating terrorism abroad is under way," a police spokesman in Manchester said, without giving further details.
Police can detain terrorism suspects for up to 14 days before charging them.
Britain has been on high alert since four suicide bombers killed 52 people in coordinated attacks on London's transport network on July 7th last year.
All four were British Muslims. Police are also on the alert for Britons they suspect of planning terrorist attacks abroad.
With a Muslim population of around 1.7 million, many angered by British foreign policy in the Middle East, Britain has been cited as a fertile recruiting ground for Islamist extremists.
In December 2001, Briton Richard Reid was arrested for trying to blow up a passenger plane with explosives hidden in his shoes.
He was sentenced to life in jail in January 2003 after a trial in which he described himself as an Islamic fundamentalist working in league with al Qaeda.
In 2003, two British Muslims attacked a bar in Tel Aviv, killing three Israelis.
One of them, Asif Mohammed Hanif, blew himself up at the scene while the other, Omar Sharif, fled, and was later found drowned in the sea nearby.
US authorities arrested nine Britons abroad in the wake of the attacks of September 11th, 2001 and imprisoned them at the Guantanamo Bay naval base in Cuba. None of them was ever charged with any offence and all have since been freed.