KABUL: Commemorations for the holy day of Ashura in the Afghan capital, Kabul, were subdued on Wednesday, amid security fears after a gunman killed at least 18 worshippers and wounded 50 during a religious gathering at a shrine.

The attacker, said to be wearing a police uniform, entered the Karte Shakhi mosque on Tuesday night and opened fire on a crowd of minority Shi'ite Muslims gathered to observe Ashura.

Among the dead were four women and two children, according to the United Nations, which condemned the attack as an "atrocity".

At least 18 civilians were killed and 50 wounded, the United Nations said. Some witnesses said the toll could be higher.

Mourners buried at least one of the victims, a four-year-old girl, on Wednesday morning.

"Last night was a doomsday for us," said Mohammed Hussain, one of the girl's relatives.

Members of Kabul's Shi'ite community were also targeted in a suicide bombing in July that killed more than 80 people and wounded 130.

"We are not happy with the government and the police," Hussain said. "They both failed to protect us and provide security for us."

Ashura, which is an especially holy day for Shi'ite Muslims, marks the 7th Century death of a grandson of the prophet Mohammed.

The day is typically marked by processions that often include self-flagellation by some worshippers.

Amid security fears and government warnings of possible attacks, however, this year's gatherings have been more subdued.

The Taliban, who have been waging a 15-year insurgency against the Western-backed government and often conduct attacks in Kabul, denied any involvement in the shooting.

The schism between Sunnis and Shi'ites developed after the Prophet Mohammad died in 632 when his followers could not agree on a successor. Sunni Muslim militants see Shi'ites as a threat and legitimate targets for attack.