By Gordon Corera
BBC security correspondent
The arrest of Abu Faraj al-Libbi could prove to be significant not just for Pakistan but also for the broader international struggle against al-Qaeda.
Pakistan had put a bounty on Libbi's head
Intelligence sources believe he is a major player in the organisation and that this could prove the biggest detention in more than two years.
He had been on a list of six "most wanted" individuals in a Pakistani poster campaign last year for his alleged involvement in two attempts to kill President Pervez Musharraf in December 2003.
Pakistan offered a reward of 20m rupees ($340,000) for information leading to his arrest.
But he was seen as not just an al-Qaeda operational commander for activities within Pakistan but also further afield.
It is thought that, after the capture of Khalid Sheikh Mohammed in Rawalpindi in March 2002, Libbi, who had been Mohammed's close associate, took over as an international operations planner for al-Qaeda.
The al-Qaeda hierarchy has always been fluid and complex so talk of him being number three in the organisation is hard to confirm, but there have been reports that he was involved in sending messages to al-Qaeda cells overseas with instructions to carry out attacks last year - including against the US.
That international aspect was emphasised in US President George W Bush's statement, when he described Libbi as a "a top general for Bin Laden.
"He was a major facilitator and a chief planner for the al-Qaeda network," he said.
"His arrest removes a dangerous enemy who was a direct threat to America and to those who love freedom."
US officials have also indicated that the arrest came through joint co-operation with Pakistani authorities.
The hope will be that Libbi's capture leads on to more intelligence about these cells and any other international networks, as well as operations in Pakistan.
That may lead to some debate over in whose custody he is placed.
Khalid Sheikh Mohammed was transferred to US custody and is now held as a "ghost detainee" at an undisclosed location.
His interrogations have played a major role in helping the US understand the 11 September 2001 plot and are heavily referenced in the report of the commission looking into that attack.
Given Libbi's international role and links to Khalid Sheikh Mohammed, the US may well want to speak to him and hundreds of suspects have been handed over by the Pakistanis to the Americans.
PAKISTAN'S KEY ARRESTS
Khalid Sheikh Mohammed, above, Rawalpindi, March 2003
Omar Saeed Sheikh, February 2002
Abu Zubaydah, Faisalabad, March 2002
Ramzi Binalshibh, Karachi, September 2002
Naeem Noor Khan, Lahore, July 2004
Khalfan Ghailani, July 2004, Gujrat
Amjad Hussain Farooqi - shot dead in September 2004
But, in this case, the Pakistani authorities have indicated to the BBC that because of his involvement in the domestic assassination plots, they intend to keep hold of him.
There will also be some hope that Libbi may have been in contact with Osama Bin Laden or Ayman al-Zawahiri and that he may help in the hunt for these two key figures.
In a BBC interview this March, President Musharraf said that his country's intelligence services had their strongest indication of the al-Qaeda leader's whereabouts about eight to 10 months earlier but that they had lost the trail.
Bin Laden last appeared in a video just before the November US election and is suspected to be hiding along the Pakistan-Afghanistan border - perhaps even the area where Libbi is reported to have been picked up.
Pakistan says that it has rounded up around 700 suspects and last summer captured and killed a number of suspected senior al-Qaeda operatives with international links - its hope will be that Libbi leads them on to some more.