Afghanistan is holding a national day of mourning for the Vice President, Haji Abdul Qadir, who was assassinated in Kabul on Saturday.

This is a black day for us

Kabul resident Ibrahim

Flags are flying at half mast across the country, and mosques are holding prayer services as a mark of respect for the Pashtun leader.

Mr Qadir was shot dead in his car, along with his son-in-law. Twelve people have been arrested, including 10 security guards.

While it is still not known who carried out the killing, correspondents say Mr Qadir's death highlights concerns about whether the government can impose order on a country that has spent more than two decades engaged in war and civil conflict.

Reports from the Afghan capital say traffic is light although most shops and markets are expected to remain open.

'Black day'

"This is a sad day of remembrance for a great man and a hero of Afghanistan," a spokesman for Afghan President Hamid Karzai said.

Qadir was the most powerful Pashtun after Hamid Karzai

"This is a black day for us...," the Associated Press quoted Ibrahim, a Kabul resident, as saying.

"Qadir was a good man and we'll pray today that he goes to heaven," he said.

On Monday the international security force (ISAF) in Afghanistan agreed to join the investigation into Mr Qadir's death, following a request from President Karzai.

The investigation is being spearheaded by a five-man commission comprising ministers, ex-ministers and the head of the intelligence.

But the BBC's Kate Clark in Kabul says finding the assassins will not be easy.

Eye-witnesses

It is five months since the Minister for Civil Aviation was stabbed to death in front of scores of witnesses at Kabul airport, but no one has been charged.

Qadir was a good man and we'll pray today that he goes to heaven

Kabul resident

And the murder of Mr Qadir looks like being a more difficult case.

But one of the commission members, Haji Mangal Hussein, said they did have eye-witnesses, and at least one man who said he would recognise the assassins if he saw them again.

The commander of ISAF, Major General Hilmi Akin Zorlu, said his soldiers were on high alert in the capital to prevent any further attacks.

"It is vital to bring the perpetrators of this crime to justice as soon as possible and ISAF will donate every resource required to achieve it," he said.

Powerful leader

Qadir was a powerful military and political figure with vast business interests in the east of Afghanistan.

The killing has added to insecurity in Kabul

An old mujahideen commander who fought against the Taleban, he was also suspected by Western diplomats of making money out of the heroin trade.

He first achieved fame during the war against the Soviets in the 1980s.

Having made his money exporting goods to Pakistan, he was the first Afghan host to al-Qaeda leader Osama bin Laden, letting him use a network of caves in the east after he left Sudan.

He went on to become one of the few prominent Pashtuns to join the Tajik and Uzbek-dominated Northern Alliance which fought the Taleban.