'No place in Islam'
At a news conference, ul-Qadri said Islam was a religion of peace that promotes beauty, "betterment", goodness and "negates all form of mischief and strife".
"Terrorism is terrorism, violence is violence and it has no place in Islamic teaching and no justification can be provided for it, or any kind of excuses or ifs or buts," he said.
A number of edicts condemning extremism have been made by Islamic groups since the September 11 attacks on the United States, but ul-Qadri insists his is the most wide-reaching.
"This is the first, most comprehensive fatwa on the subject of terrorism ever written," he told the Reuters news agency.
"I have tried to leave not a single stone unturned on this particular subject and I have tried to address every single question relevant to this subject."
Pakistan-born ul-Qadri, 59, has written about 350 books on Islam, and is a scholar of Sufism, a Muslim branch that focuses on peace, tolerance, and moderation.
The Quilliam Foundation, a UK counter-extremism think-tank, said the fatwa was "arguably the most comprehensive" theological refutation of Islamic extremism.
Tim Winter, a lecturer in Islamic studies at Cambridge University, said while ul-Qadri's step of declaring "miscreants as unbelievers" was unusual, it was unlikely extremists would take notice of his edict.
"Those who are already hardliners will pay no attention at all. But 'swing voters' - poorly educated and angry Muslims, who respect mainstream scholars, will probably take note," he told Reuters.
Ul-Qadri said he felt compelled to issue the edict because of concerns about the radicalisation of British Muslims at university campuses and because there had been a lack of condemnation of extremism by Muslim clerics and scholars.
The Minhaj-ul-Quran movement, founded in Pakistan in 1980, works around the globe to promote peace and interfaith dialogue.