His killing deals a heavy blow to already remote hopes of ending 10 years of fighting between insurgents and the Afghan government.
Hundreds of Afghans marched in Kabul on Wednesday to mourn Burhanuddin Rabbani, chairman of the government's peace council, whose assassination threatens to plunge the country into fresh turmoil.
Rabbani, president during Afghanistan's 1992-96 civil war and a warlord with a chequered human rights record, was killed at his home Tuesday by a bomber thought to be a trusted emissary bringing a special message from the Taliban.
Although there has been no communication from the Taliban on the attack, his killing deals a heavy blow to already remote hopes of ending 10 years of fighting between insurgents and the Afghan government backed by Western troops.
President Hamid Karzai was rushing back to Kabul from a visit to the United States, while an emergency cabinet meeting was held to discuss plans to give Rabbani an official funeral in the coming days, officials said.
Haji Shabudin, head of the secretariat of Rabbani's Jamiat-i-Islami party, confirmed the ceremony would take place Friday in Kabul.
A source at the presidential palace speaking to AFP on condition of anonymity said it would be a state funeral and there would be three days of national mourning in Afghanistan from Thursday.
Rabbani's killing was the most high-profile political assassination in Afghanistan since the 2001 US-led invasion dislodged the Taliban.
That it happened in Kabul's supposedly secure diplomatic zone, close to the scene of last week's 19-hour siege targeting the US embassy, again highlights a sharp rise in spectacular Taliban attacks in Afghanistan.
Under heightened security on Wednesday, several hundred mourners marched to Rabbani's home carrying giant pictures of him and wearing black headbands.
Supporters recited verses from the Koran while a string of government officials arrived to pay their respects, an AFP photographer said.
Police, who blamed the Taliban for the killing, stepped up security in the area, preventing cars from entering and searching many pedestrians.
A string of events to mark today's International Day of Peace in Kabul were cancelled for security reasons, including a free concert for women by leading Afghan singer Farhad Darya.
Rabbani led the High Peace Council which Karzai established last year and charged with establishing contacts with insurgents.
But his efforts so far appeared to have come to little despite growing international hopes of a settlement.
Karzai, whose relations with the West have soured badly since his fraud-tainted re-election in 2009, insisted Rabbani's assassination "will not deter us from continuing down the path we have started".
US President Barack Obama, who has said American combat troops will leave Afghanistan by the end of 2014, said Afghans must be allowed to live "in freedom, safety, security and prosperity".
NATO's Anders Fogh Rasmussen, who heads up the alliance leading the foreign military effort to reverse Taliban momentum, said those behind the killing "will not prevail".
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