The deadly attack on the police headquarters in Kandahar city took place over several hours [Reuters]

At least 21 people have been killed following an attack by Taliban fighters on the police headquarters in Kandahar city, in southern Afghanistan, sources say.

The attack on Saturday, which took place over several hours and left at least 45 people injured, involved several suicide bombers armed with guns and grenades, provincial officials said.

The attackers fired on the main police headquarters after occupying a wedding hall opposite.

Initial reports suggest a car bomb exploded outside the police compound.

Immediately afterwards, two suicide bombers tried to storm the headquarters but blew themselves up outside the perimeter wall, according to NATO officials in Kandahar.

Other reports said that three car bombs were detonated near the police office and a further three were defused before they could go off.

The exact number of casualties was unclear.

"Fifteen people were killed and 45 others suffered injuries and some of the wounded are in critical condition," Tooryalai Wesa, the Kandahar governor, said.

He later clarified that the 15 killed were all police. Six suicide bombers were also said to have been killed in the attack.

Taliban claim

Zemeri Bashary, an interior ministry spokesman, said that three Afghan soldiers, two civilians and one intelligence service agent were also killed.

The Taliban claimed responsibility for Saturday's attack.

"We sent six men to the building, they attacked police headquarters and blew themselves up," Yousuf Ahmadi, a spokesman for the group, said.

"We also detonated six explosive-packed cars outside  the police headquarters."

The raid indicates the Taliban is still able to launch deadly attacks on government institutions despite the past year's influx of US troops into Kandahar, the group's birthplace.

Despite the build-up of foreign troops, the province has been hit by frequent attacks.

Earlier this week, a US customs officer died in an attack on a customs office in Kandahar, while last month, Abdul Latif Ashna, the deputy governor, was killed on his way to work by a suicide bomber.

The attack was swiftly condemned by Hamid Karzai, the Afghan president, who said the "enemies of Afghanistan" had "once again revealed their evil purposes against this country".