Unknown attackers killed the cleric, who ran an Islamic seminary in northwestern Pakistan and was seen as a possible intermediary in talks between the Afghan government and the Taliban, his deputy Yousaf Shah said.
There were conflicting reports of exactly how he was killed and why his bodyguard and driver were apparently not there to defend him at the time of the attack.
Shah initially said that Haq had been shot dead.
Haq's nephew Mohammad Bilal told Reuters that his uncle was found with stabbing and gunshot wounds in a house he owns in an upscale area on Islamabad's outskirts. "When the assailants entered his house ... They first started hitting Mullah Sami ul-Haq with knives and daggers and then shot him dead," he said.
Further details remained unclear.
One of his students from the 1980s, known later as Mullah Mohammad Omar, went along with classmates to Afghanistan to join mujahideen groups fighting against the Soviet occupation of the country.
"Recently, when the Afghan government sent him a delegation and sought his help in bringing the Afghan Taliban to the negotiation table, he offered them ... his madrassa (seminary) to sit with each other and build trust," a member of Haq's family told Reuters.
The ultra-conservative Taliban imposed an extreme version of Islamic sharia law on Afghanistan that included forbidding women to leave home without a male relative, imposing minimum lengths on men's beards and banning sports, radio and television.
Haq's seminary has continued to thrive in Pakistan, including being allocated funding in the budgets of the provincial government, which is headed by Imran Khan's Pakistan Tehreek-e-Insaf (PTI) party. Haq had emerged as one of Khan's allies since the PTI came to power in the province after elections in 2013.
Pakistan's Interior Ministry confirmed Haq's death in a statement on Friday evening and expressed its condolences.
A spokesman for the military condemned the "assassination" and expressed "grief and condolences" to his family.
Hundreds of protesters took to the streets near the northwestern town of Mardan near Haq's seminary, setting a highway toll station on fire. (Reporting by Jibran Ahmed; Writing by Kay Johnson; editing by David Stamp and Hugh Lawson)
(This story has not been edited by Devdiscourse staff and is auto-generated from a syndicated feed.)