More than 3,000 people were attending the rare "loya jirga", which is being billed as the largest in modern Afghan history, in a bid to set possible conditions under which they might accept a peace settlement.

The loya jirga -- literally "grand assembly" in Pashto -- is being held over four days as the US and Taliban are discussing a possible foreign troop withdrawal from Afghanistan in exchange for a permanent ceasefire and various Taliban pledges.

"We want to specify the main lines for the negotiations with the Taliban," Afghan President Ashraf Ghani said at the start of the summit. "We want clear advice from all of you."

The jirga is seen as an attempt by the Afghan government to influence the peace talks between the US and the Taliban aimed at ending more than 17 years of conflict.

So far, however, the talks have cut out Ghani, whom the Taliban view as a US stooge.

"Come to any part of the country for talks, why don't you want to talk to Afghans?" Ghani said, addressing the Taliban, who have boycotted the jirga.

"We are ready to talk to you without any preconditions."

- High-level boycotts -

Ghani's government hopes the high-stakes meeting will define Kabul's conditions for any deal, including the continuation of the constitution and the protection of women's rights, the media, and free speech.

But some prominent Afghans, presidential hopefuls and government officials including the country's chief executive Abdullah Abdullah have boycotted the assembly.

They have criticised it as an attempt by Ghani to boost his standings ahead of presidential elections slated for September.

Much of Kabul was locked down Monday, with a week-long public holiday declared for the duration of the four-day event.

Streets across the capital were closed and hilly overlooks blocked. In the past, the Taliban have blasted rockets at a tent hosting a loya jirga.

In a statement, the Taliban have vowed that any decisions or resolutions made at a loya jirga are "never acceptable to the real and devout sons of this homeland".

The insurgents now control or influence about half of Afghanistan. They announced the start of their annual spring offensive early this month, despite talks with US representatives -- dashing hopes for a quick ceasefire in the war-ravaged country.

The loya jirga is a centuries-old tradition in Afghanistan that has been convened at times of national crisis or to settle big issues.

The most recent jirga was held in 2013, when Afghan officials endorsed a security agreement that allowed US troops to stay in Afghanistan beyond their planned withdrawal in 2014.

Delegates this year have split into various committees to discuss negotiating points for future talks with the Taliban. The summit's results are due to be announced Thursday.