Officials with Nato's International Security Assistance Force (Isaf) said they were aware of the attack but did not have any details.
Hashem Ahelbarra, Al Jazeera's correspondent in Afghanistan, quoted a Taliban spokesperson as saying that two suicide bombers blew themselves up near the entrance of the gate to the prison before 30 Taliban fighters attacked and killed 16 policemen.
"The Taliban spokeperson said they had freed 400 Taliban fighters and there were around 1,300 prisoners in the jail, including common criminals," he said.
"It was a co-ordinated attack and they [Taliban fighters] got assistance from the guards within the jail."
A police official, speaking on condition of anonymity, said: "There was an explosion at the main gate of the prison, apparently a car bomb, followed by an assault by a number of Taliban with small arms fire and rocket-propelled grenades."
"They went into the prison and freed a number of political prisoners.
|Some 400 Taliban prisoners are believed to
have escaped during Friday's attack [AFP]
"There's still sporadic fire inside the prison; I don't know how many people attacked and how many prisoners were freed."
An Afghan intelligence official said separately there had been at least one explosion and that some prisoners were believed to have escaped.
Afghan and foreign troops were outside the prison, the intelligence official said.
Kandahar jail was the scene of a mass hunger strike by hundreds of inmates in May, during which 47 of the prisoners sewed their lips shut after complaining they had been tortured and denied fair trials.
Prisoners, including al-Qaeda and Taliban fighters, rioted at the country's main Pul-e-Charki jail in Kabul in February 2006, taking control of a block and leaving several people injured.