At least 20 people have been killed in a suspected suicide bomb attack on a packed mosque in the southern Afghan city of Kandahar.
There have been conflicting claims about who carried out the attack
Kabul police chief Mohammed Akram was among those killed, officials say.
He was among many who were at the mosque to mourn a senior anti-Taleban cleric, who was shot dead on Sunday.
About 40 others were hurt in the blast, the worst act of violence Afghanistan has seen this year. President Karzai called it an act of cowardice.
"A mosque is a home of God and place of worship. Those attacking it are unbelievers and despicable enemies of Islam and Afghanistan," he said in a statement.
Many ordinary Afghans were outraged at the bombing - it is extremely rare in Afghanistan for mosques to be the target of attacks.
Level of anger
Kandahar Governor Gul Aga Sherzai has alleged that Arab militants belonging to Osama Bin Laden's al-Qaeda network were behind the attack.
"The attacker was a member of al-Qaeda. We have found documents on his body that show he was an Arab," Mr Sherzai is quoted as saying by the Associated Press.
A man claiming to speak for the Taleban telephoned the BBC in Kabul to say his organisation had carried out the attack.
However, another Taleban spokesman, Latifullah Hakimi, later denied the group had been involved.
The BBC's Andrew North in Kabul says the denial is impossible to verify, just as it was impossible to check the earlier claim of involvement.
But he says the hardline movement has claimed previous suicide attacks in Afghanistan and some analysts believe the Taleban may have decided to deny involvement in the latest blast after gauging the level of anger it has provoked.
Eyewitnesses say the Abdul Rab mosque in the heart of Kandahar was filled with people who had gathered to mourn murdered cleric Mawlavi Abdullah Fayaz when the blast ripped through the building at 0900 (0430 GMT).
Mohammad, who owns a money-changing shop in the vicinity of the mosque, told the BBC News website: "The force of the blast blew out the windows of my shop."
Sirens could be heard from the blast site as ambulances ferried the wounded to hospital.
"People were running around, some were lying on the ground crying," one survivor, Nanai Agha, told the Associated Press.
"Dead bodies were everywhere," he said.
"It was a suicide attack by the enemies of Afghanistan and Islam," an Afghan interior ministry spokesman told AFP. "The investigation into the case has started."
Mawlavi Abdullah Fayaz was killed by two men on a motorcycle as he left his office on Sunday. Last week he made a speech attacking Taleban leader Mullah Omar.
Mawlavi Abdullah Fayaz was a key supporter of Afghan President Hamid Karzai and was head of the government-appointed Islamic scholars' council.
Last month, he had condemned the Taleban at a meeting in Kandahar of about 500 clerics.
He said Taleban fighters were killing innocent civilians and the government should be supported for trying to rebuild the country.
Our correspondent says that the latest attack will raise fears that militants opposed to President Karzai are stepping up their efforts to undermine his government ahead of September's parliamentary elections.