Khalid Sheikh Mohammed - the alleged al-Qaeda mastermind arrested last weekend in Pakistan - is a "hero of Islam", Pakistan's largest Islamic party has said.
Jamaat-e-Islami members condemn FBI arrests and the build-up to war in Iraq
A spokesman for Jamaat-e-Islami, Amirul Azeem, said the Pakistani Government, acting on US orders, had committed a "shameful sell-out".
The outspoken endorsement of Sheikh Mohammed is an indication of the difficulties Pakistani leader Pervez Musharraf may face in reconciling his urge to assist US anti-terrorism efforts with rising anti-American sentiment at home.
Regional scorn for such support was reflected in a message sent to the BBC from a militant Kashmiri group condemning Islamabad's decision to hand him over to the US.
Mr Azeem said he applauded al-Qaeda's defiant attitude to the US.
"Those who fought jihad [holy war] in Afghanistan... who refused to be dictated to by the Americans are heroes of Islam," he told Reuters news agency.
"These man are the targets of America, but Pakistanis consider them their guests - they are ready to give them refuge.
"The government is acting on the dictates of America. This shameful sell-out is not acceptable to the people."
Sheikh Mohammed's arrest is said to be a body blow for al-Qaeda
He made similar comments in interviews with other news agencies.
Jamaat-e-Islami is one of six hardline Islamic groups who, as an alliance, have condemned US-led efforts to fracture terrorist networks in the region.
Recent elections and deals with other parties have seen Muttahida Majlis-e-Amal (MMA), as the alliance is known, gain significant political influence.
In another development, the BBC office in Srinagar, Indian-administered Kashmir, received a fax from a militant group strongly condemning Sheikh Mohammed's deportation.
Al-Nasireen, believed to be a front for a major militant group fighting Indian rule in Kashmir, said Pakistan's decision reflected its anti-Islamic policy.
The decision, it said, defied teachings of Prophet Mohammad which said Muslims should provide shelter and refuge to fellow Muslims.