Reuters raw footage of the scene of the attack
UPDATE 1400 GMT: Afghanistan's Pajhwok News puts the death total at three, with 15 injured. One of the dead are clerics Maulvi Hikmatullah Hikmat, the head of Kandahar Ulema Council.
The attacker entered the mosque hosting a prayer gathering for the slain brother of President Karzai and detonated explosives strapped to his body.
Interior Ministry spokesman Ghulam Siqiq Sidiqi had said four people were killed.
CBS and Associated Press report:
A suicide blast tore through a packed mosque in the southern Afghan city of Kandahar on Thursday as mourners offered prayers in a service for Ahmed Wali Karzai - the region's most powerful man and a half-brother of the Afghan president - who was murdered at his home days earlier.
CBS News' Fazul Rahim says Afghanistan's Interior Ministry confirmed the blast and said there were at least four people killed, but the number and extent of the injuries was changing rapidly as the situation unfolded.
There were reports of a second explosion even as people surveyed the damage from the first blast, but few details were available and police would not confirm the reports.
The service for Wali Karzai was a second memorial, following a formal funeral on Wednesday when President Hamid Karzai climbed into his half-brother's grave and beseeched insurgents in Afghanistan to stop killing their fellow countrymen.
Interior ministry spokesman Sediq Seddeqi told Rahim it was a suicide attack carried out inside the mosque.
At least four people were killed in the blast, according to Seddiqi, including a prominent religious scholar and head of a religious council for Kandahar. At least 11 others were injured in the explosion.
Seddiqi said none of the president's family members appeared to have been injured in the attack.
There was no immediate claim of responsibility for Thursday's bombing.