The arrest came a day after top U.S. law enforcement officialsindicates a major terror attack could occur in the summer, and the FBI posted a list of seven wanted al Qaeda operatives.
Al-Masri is not among the seven wanted figures, but has been the focus of terror suspicions for years in Britain. The Egyptian-born cleric formerly preached at a mosque linked to several terrorist suspects and has been accused by the government of advising and supporting terrorist groups, including al Qaeda.
London's Metropolitan Police press office refused to confirm the detention, saying only that officers from the Extradition and International Assistance Unit arrested a "British citizen, aged 47" at about 3 a.m. Thursday following an American request for his extradition. It said he was to appear at Bow Street Magistrates' Court later Thursday.
The force said it didn't know what allegations against al-Masri were contained in the U.S. extradition request. The U.S. Embassy in London said it doesn't comment on ongoing investigations.
Police sources, speaking to The Associated Press on condition of anonymity, confirmed that al-Masri was the man detained.
Al-Masri's lawyer, Maddrassar Arani, told British Broadcasting Corp. radio that she had spoken to her client, who was being held in a central London police station.
"He was quite calm about it," Arani said. "He said take your time and come down whenever you can."
She added that she didn't know what the charges were against al-Masri as she hadn't seen a copy of the warrant.
Anti-terrorist officers also conducted a search of al-Masri's west London home, police said.
The Home Office declined to comment on the arrest.
Al-Masri is one of Britain's best-known Islamic radicals. He has been fighting deportation by the government. He is also wanted in Yemen on charges of orchestrating terrorism there from Britain.
In April, an FBI official told Congress that the U.S. had in 2002 detained Earnest James Ujamma as a material witness in part because he tried to start a terrorist training camp in Oregon as part of "Hamza's jihad recruiting program, and served as Hamza's representative for the delivery of cash, computer hardware and software, and at least one combatant to Taliban and al Qaeda activities inside Afghanistan."
The British government revoked his British citizenship in April 2003, calling him a threat to the country's interests. He has appealed that decision to a special immigration tribunal and a ruling isn't expected until early next year.
At an immigration hearing last month, a government lawyer said al-Masri had "provided advice and support to terrorist groups," including al Qaeda and the Islamic Army of Aden, the organization that claimed responsibility for the bombing of the USS Cole in Yemen in 2000.
The lawyer said al-Masri had encouraged others to engage in jihad, "including fighting overseas and engaging in terrorist acts."
Al-Masri, who married a British woman and took British citizenship in 1981, denies any involvement in violence and says he is only a spokesman for political causes.
The fiery preacher with one eye and hooks for hands — lost, he says, fighting Soviet troops in Afghanistan in the 1980s — is a tabloid hate figure in Britain.
The former head preacher at Finsbury Park Mosque in London, al-Masri has sparked outrage with sermons calling the invasion of Iraq a "war against Islam," claiming the Sept. 11 attacks were a Jewish plot and calling the Space Shuttle Columbia disaster a "punishment from Allah" because Christian, Jewish and Hindu astronauts were aboard.
The Finsbury Park Mosque was been linked to several terrorist suspects, including Sept. 11 suspect Zacarias Moussaoui and "shoe bomber" Richard Reid.
The mosque was shut down by its trustees after a police anti-terrorist raid in January 2003. The next month al-Masri was banned from preaching there by a government body because his "extreme and political" statements conflicted with the mosque's charitable status.
Since then the cleric has led Friday prayers on the street outside, under the watch of police.