The blast occurred at the Khalifa Sahib Mosque in western Kabul after Friday prayers.
According to Sayed Fazil Agha, a suspected suicide bomber attended the ceremony at the mosque.
Afghanistan's Interior Ministry said earlier in the day that at least 10 people had been killed and at least 15 injured.
However, information to Reuters from sources in the health care system indicates that the number of dead and injured is significantly more.
It is not clear what caused the explosion, but the terrorist group IS has carried out a number of attacks on Shia Muslims in Afghanistan in recent weeks.
At least nine people were killed on Thursday when two bombs exploded on two different minibuses in Mazar-e-Sharif in northern Afghanistan.
Twelve people were killed in a bomb attack on a Shia mosque in Mazar-e-Sharif last Thursday, an attack carried out by the Sunni Muslim terrorist group IS.
Last Friday, an explosion occurred in a mosque with an associated religious school in Imam Sahib in Kunduz province.
33 people, many of them students at the school, were killed then.