The main challenger to the Afghan president said Sunday he would boycott a traditional leaders' assembly to discuss peace moves and ties with the United States, calling it illegal and unconstitutional.

President Hamid Karzai has convened the assembly, or loya jirga, this week to discuss strategies for trying to broker peace with Taliban insurgents and long-term relations with the United States after combat troops leave.

Abdullah Abdullah, rival to Karzai in the runoff round of the 2009 presidential election, told a news conference in Kabul: "I won't participate in the upcoming traditional loya jirga. Holding this... doesn't have any legal base, and it is contrary to Afghanistan's constitution."

He did not rule out attendance of the assembly, to be held from November 16, by members of the opposition Change and Hope Coalition, which he heads.

The loya jirga follows the September assassination of Karzai's chief peace negotiator Burhanuddin Rabbani by a bomber who purported to be a peace envoy for the Taliban, amid government attempts to negotiate with the insurgents.

Abdullah also denounced vagueness surrounding the future of Afghanistan's partnership with the US -- which led the 2001 invasion that ousted the Taliban from government -- after the planned withdrawal of its combat troops by the end of 2014.

"I have not seen any documents" on the future partnership, said Abdullah, who also served as foreign minister under Karzai from 2001 to 2006.

A deal still under negotiation with the Americans will govern the longer-term presence of US troops in Afghanistan, a highly sensitive issue in Afghanistan and the wider region.

The loya jirga, which brings together representatives from Afghan provinces, tribes, ethnic groups and civil society, is held on rare occasions to discuss major policy decisions.

Another coalition recently created around two former warlords, Abdul Rashid Dostum and Haji Mohammad Mohaqeq, has also denounced the planned assembly.

Lawmakers had voiced concern it would be used to bypass their powers, but a spokeswoman for the assembly said Saturday its decisions would not be binding and parliament would have the final say.

More than 2,000 people are expected to attend the meeting in a giant tent in Kabul.