The US has placed a $5m (£3.6m) bounty on the head of Haqqani, who attended the Pul-e-Kheshti Mosque, where reports say hundreds pledged their allegiance to the Taliban.
He is part of the hardline Haqqani network and was pictured among the large crowds in the Afghan capital on Friday, flanked by armed men dressed in military uniforms.
The Haqqani network is one of the region's most powerful and feared militant groups, and has been credited with some of the most violent attacks against Afghan forces and their Western allies in recent years.
Earlier in the week, the head of Afghanistan's High Council for National Reconciliation, Abdullah Abdullah, posted photos of a meeting with Haqqani and his Taliban delegation, as well as the former Afghan president Hamid Karzai.
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Cover image: Sea Knight military transport helicopter flies over Kabul in July (Picture: UPI/ALAMY).