Haq, 82, was the head of the Islamic religious seminary Darul Uloom Haqqania in Khyber Pakhtunkhwa's Akora Khattak town and also the chief of the hardline political party Jamiat Ulema-i-Islam-Sami (JUI-S).
Haq, a heart patient, was killed by unidentified attackers while he was resting in his room, his son Maulana Hamidul Haq was quoted as saying by Geo News.
His personal guard had gone out to the market and when he came back he saw Haq lying "in a pool of blood" on the bed, Hamidul added.
JUI-S's Peshawar president also confirmed Haq's death following an assassination attempt in Rawalpindi.
Initially there were conflicting reports about how Haq was killed. Some Pakistani media had reported that he was killed in a gun attack.
Haq's son has clarified that the cleric was attacked with knife.
Mulana Abdul Majid Hazarwi, a close associate of Haq, also confirmed that he was attacked with knife.
So far, no outfit has claimed the responsibility of the killing.
A probe has been launched to nab the assailants who apparently escaped on a motorbike, police said.
Haq's funeral services will be held at 2:00 PM on Saturday in Akora Khattak.
The cleric was elected twice to Pakistan's parliament on Islami Jamhoori Itehad ticket.
He was also the chairman of the Difa-e-Pakistan Council -- an umbrella coalition of more than 40 groups, including Hafeez Saeed-led Jamat-ud Dawa (JuD) and the banned Sipah-e-Sahaba.
Haq's madrassa in Akora Khattak is known for having several top Afghan Taliban leaders among its alumni, including Mullah Omar who had received an honorary doctorate from the seminary.
Haqqani Network founder Jalaluddin Haqqani, Al-Qaeda in the Indian Subcontinent (AIQS) leader Asim Umar and slain Afghan Taliban chief Mullah Akhtar Mansoor were also among the alumni of the seminary which is dubbed as the "University of Jihad".
However, Haq never supported militant activities in Pakistan.
He was an ally of Prime Minister Imran Khan and supported the anti-polio vaccination drive by his government when hardcore militants opposed the campaign by terming it as a "western effort to sterilise Muslims".
Meanwhile, Khan, who is in China on an official visit, condemned the killing of Haq and ordered the authorities to prepare a report of the incident and submit it to him.
Former premier Nawaz Sharif also condemned the killing of the leading cleric and urged public to remain clam.
Pakistani Army chief General Qamar Javed Bajwa also condemned the "assassination of the renowned religious scholar and political leader".
Pakistan's Minister of State for Interior, Sheryar Afridi, condemned Haq's killing and said, "The political and religious services of the Mualana will be remembered forever."
Maulana Fazlur Rehman, chief of Jamiat Ulema-i-Islam- Fazl (JUI-F), also condemned the attack. "I am with the family of the Maulana at this difficult hour," he said.