UPDATED REPORT - Pakistan said it had arrested the suspected mastermind behind the September 11 attacks, Khalid Sheikh Mohammed, today in a major breakthrough in the international crackdown against Osama bin Laden's network.
"We have finally apprehended Khalid Sheikh Mohammed," Rashid Qureshi, spokesman for President Pervez Musharraf, told Reuters.
"It was the work of Pakistani intelligence agencies... It is a big achievement. He is the kingpin of al Qaeda."
In June 2002, US investigators identified Mohammed as the probable mastermind behind the September 11 attacks on New York and Washington.
Mohammed was one of three people detained in a raid on a house near Islamabad.
Officials said all three were taken by surprise and that there was no shootout. Two of those detained were foreigners - Mohammed is from Kuwait - and the other was a Pakistani.
Mohammed was indicted in the United States in 1996 for his alleged role in a plot to blow up American civilian airliners over the Pacific.
Believed to be in his late 30s, the United States put a $25 million price tag on Mohammed's head and the FBI posted him on its "most wanted" list of 22 individuals in October 2001.
A senior US official in Washington described the capture as "extraordinarily significant" because he was critical to planning future assaults on the United States.
"He's the senior-most al Qaeda guy to be captured yet," the official told Reuters. "He was critical to not only the planning for the September 11, 2001, attacks but also is central to the planning for future attacks."
In Washington, FBI spokesman John Iannarelli said: "The FBI is aware of ongoing reports and our official position is we are not going to add any additional information at this time."
He said anybody arrested who was on the FBI's top 20 list would be important.
Mohammed is a relative of Ramzi Ahmed Yousef, now serving a life sentence for involvement in a 1993 bombing of New York's World Trade Centre that was destroyed in the September 11 attacks.
"This is a big success for Pakistan. He is the most wanted man in the world. It was a very successful operation," Pakistan Information Minister Sheikh Rasheed Ahmad told CNN.
Pakistani security agencies have been hunting al Qaeda members with the help of US intelligence agents since the ousting of the hardline Islamic Taleban government in neighbouring Afghanistan in late 2001.
Right-wing Islamic parties in Pakistan have strongly opposed the involvement of US Federal Bureau of Investigation (FBI) agents in past raids on al Qaeda suspects, but Pakistani officials said the FBI was not involved in Saturday's search.
Hundreds of al Qaeda militants and their Taleban allies are believed to have crossed into Pakistan since US-led forces began hunting for them in Afghanistan after the end of Taleban rule.
Pakistan says it has arrested more than 400 suspected Islamic militants since late 2001, but Mohammed is by far the biggest catch yet.
Other al Qaeda figures that Pakistan has arrested include Ahmed Omar Abdel Rahman, known as Binalshibh in the West, who was a key figure in the Hamburg cell suspected of carrying out the September 11 attacks. He is now in US custody
Although Pakistan's information minister said that Mohammed would be extradited to the United States, the official told Reuters that was not the most likely plan.
He said the alleged al Qaeda operative was expected to be handed over to US custody but was "unlikely" to be legally extradited to the United States.
Instead, "he will probably go the route of other top al Qaeda leaders like Abu Zubaida," who was captured in Pakistan and has been interrogated by US authorities at an undisclosed location overseas.
"There is a difference between US custody and extradition," the official said. Extradition is a legal term implying that a suspect would be subject to US laws and protections.
The Bush administration has made clear its determination after the September 11 attacks to use extraordinary means if necessary to interrogate and bring to justice alleged "terrorist" suspects.