Many others were wounded in the attack in Tirin Kot, the capital of Uruzgan province in the south of the country.

The explosion, which was described as "very powerful", also damaged several buildings, police said.

Southern Afghanistan has been the centre of a Taleban-led insurgency since the US invasion in 2001.

'Contempt'
 

Officials say the attack is the worst on the police in recent months in Afghanistan.

"As a result of the suicide attack on a police unit... 21 police were martyred and eight more wounded," the interior ministry said in a statement.

About 10 other policemen were wounded but released from hospital after being treated, said Juma Gul Himat, the Uruzgan provincial police chief.

He said the bomber had entered the police compound as a group of reservists was training and detonated explosives strapped to his body under his uniform.

A Taleban spokesman said his group had carried out the attack. Uruzgan is the home province of the movement's leader Mullah Mohammad Omar.

UN special representative to Afghanistan Kai Eide said the attack showed "contempt both for human life and for the community's wishes for a just Afghanistan".
 

Taleban tactics
 

Separately, a spokesman for the Afghan Defence Ministry told the BBC that its soldiers had arrested three men wearing suicide vests in Dirawoad district near to Tirin Kot.

Afghan police and soldiers, as well as US and other foreign troops belonging to the Nato-led International Security and Assistance Force (Isaf), are frequent targets of Taleban attacks.

The Taleban have changed tactics since facing foreign troops in open battles two years ago, says the BBC's Ian Pannell in Kabul.

The tactics of insurgents in Iraq are being duplicated, with more suicide bombings, roadside bombs and hit-and-run ambushes, our correspondent says.

Uruzgan is not as violent as Kandahar or Helmand provinces, adds our correspondent, but the number of attacks there has been growing.
 

Police recruitment
 

General Kai Vittrup, head of the EU police mission in Afghanistan, said that despite police being a target for suicide bombers, there was no problem with recruiting local people.

"Of course an incident like this is always a sad thing, but there is only one thing to do - that is to move forward to win this war," he told the BBC.

"If we just run away, if the Afghan police run away, it would be the same as surrender. And that is certainly not what is going to happen".

Southern Afghanistan is the main battlefront between the insurgents and Afghan and foreign forces, but there have been attacks elsewhere in the country, notably in eastern areas and also in the capital, Kabul.

As violence in Iraq subsides, the US is considering sending up to 30,000 more troops to Afghanistan.