He vowed to "protect the independence and territorial integrity of Afghanistan in the name of God".

Mr Karzai will lead the new transitional government until elections in 18 months.

The inauguration was followed by the closing of the assembly - the loya jirga - which has been meeting for more than a week to discuss the country's future.

Mr Karzai announced his cabinet in a pre-inauguration speech, which the loya jirga then approved the choices by a show of hands.

There's no government in any part of the world which satisfies everyone but we have tried to create a government that everyone can accept

Hamid Karzai

The defence and foreign affairs portfolios were retained by the mainly Tajik Northern Alliance, while the Interior Ministry went to an ethnic Pashtun regional governor.

The BBC's Kate Clark says the alliance appears to be the immediate winner, in particular the defence minister, Mohammad Qasim Fahim, who has also been given one of three vice-presidential posts.

There was a mixed reaction from delegates, our correspondent says, with some saying they were pleased with the ethnic balance and others fearing a government dominated by armed factions was doomed to fail.

Unity pledge

After promising earlier to provide a line-up acceptable to the assembly, Mr Karzai asked delegates to forgive him for not being able to satisfy everyone.

The loya jirga has run over by three days

Mr Karzai pledged to keep Afghanistan under one leadership, bring all regional commanders under the Defence Ministry and introduce "forceful Islamic government".

He urged the delegates to remind him of his pledges, saying he would resign if he failed to stand by them.

"If I cannot do this I will resign," he said. "I will not accept a government that is useless."

The speech comes amid reports that the UK defence minister is due to announce that British troops serving in Afghanistan will not be replaced when they leave in the summer.

The troops are among the thousands of International Security Assistance Force (Isaf) members stationed in and around the capital, Kabul, to help maintain security, and are also involved in the hunt for al-Qaeda and Taleban forces.

Swearing in

Mr Karzai arrived to a red carpet reception and honour guard armed with rifles and traditional swords.

There have been heated exchanges at the loya jirga

Mr Karzai was sworn in formally as head of state - three days earlier than planned - to take advantage of the fact that the loya jirga is still sitting.

He said the Defence Ministry would remain under the control of Marshal Fahim, the former Northern Alliance commander-in-chief, while Dr Abdullah remains as foreign minister.

Taj Mohammed Wardak, the relatively unknown Pashtun governor of Paktia province - scene of recent fighting with a rival warlord - has been made interior minister following the resignation of another key Northern Alliance figure, Yunis Qanooni, last week.

Mr Karzai appointed his adviser Ashraf Qani Ahmadzai - a Pashtun currently in charge of the aid effort - as finance minister.

Mr Qanooni has now been appointed education minister.

Mr Karzai also announced three vice-presidents - all of whom are from armed factions - to very little applause.

Leader's charm

As well as needing to win backing for his new cabinet from the country's rival warlords, Mr Karzai has to convince foreign donors that his government meets international standards.

Main appointments

Interior: Taj Mohammad Wardak Foreign: Dr Abdullah Defence: Mohammad Fahim Finance: Ashraf Qani Planning: Mohammad Mohaqeq Vice-presidents: Mohammad Fahim Haji Qadir Karim Khalili

Our correspondent says the fact that he is a civilian is both a strength and a weakness after Afghanistan's long and bitter civil war.

He has managed to charm weary delegates at the loya jirga into laughter and applause but he lacks real military support among the powerful warlords.

Men like Marshal Fahim took positions in the interim government or assumed control in the country's various regions after helping to defeat the Taleban.

Meanwhile members of the loose coalition of Afghan democrats said they had received death threats after proposing an alternative cabinet list which did not include such people.

"A man telephoned and said if we presented our alternative cabinet list he would cut me up into little pieces and send them to my family," one delegate told the BBC.