A suicide bomber attacked a religious gathering in the Afghan capital, killing at least 50 Muslim scholars and clerics who had gathered to mark the birth anniversary of the Prophet Muhammad.
The victims included religious delegates from various parts of Afghanistan, Interior Ministry spokesman Najib Danesh said.
He said the gathering was convened by the Afghan Ulema Council, the country's largest religious body, and was attended by hundreds of Sunni Muslims.
At least 72 attendees were wounded, the Health Ministry said.
In a statement, President Ashraf Ghani described the attack as "unforgivable and a clear act of hostility against Islam's teaching".
He declared tomorrow a national day of mourning and ordered that flags be flown at half-staff. The United Nations mission in Afghanistan said it was outraged by the attack.
According to government officials, the bombing was carried out by a lone assailant who targeted the ceremony in a large hall on the first floor of a hotel close to the Interior Ministry, a venue usually used for weddings.
More than 20 of those wounded in the attack were in critical condition, Health Ministry officials said.
There were unconfirmed reports about the deaths of a number of top religious figures.
Officials said they feared that the death toll could rise.
One police official said high-grade explosives were used in the blast, which left body parts strewn all over the floor of the hall. The bomber detonated his suicide vest as the scholars and clerics began to recite verses from the Koran, officials said.
"It was the beginning of the function when the blast occurred," said Shah Mohammad, a witness who helped take casualties to a hospital. "There were bodies all over the place."
Police were not asked to provide security for the event, a police spokesman said, and the suicide bomber was able to slip into the hall undetected.
No group immediately claimed responsibility for the attack.
But suspicion immediately fell on Isis (Islamic State), which is considered even more ruthless than the Taliban insurgency that has been battling the Afghan Government and its foreign backers for 17 years.
While it remained unclear who carried out today's bombing, hardline Sunnis view venerating the Prophet's birthday as sacrilegious. Although the anniversary is widely celebrated in the Islamic world, it is a holiday that extreme fundamentalists are trying to stamp out.
The Taliban "strongly condemns attacks on civilians and sessions of ulema," or Muslim clerics, Taliban spokesman Zabiullah Mujahid said in a statement.
Without naming any group, but indirectly alluding to Isis, he blamed the attack on "seditious circles" that he said have brutally killed Muslims over minor differences. He said this must be prevented.
The Taliban often uses suicide bombings in its own attacks on government and foreign targets and has been blamed for the deaths of thousands of civilians, the United Nations has reported.
In June, at least 14 people, including seven clerics, were killed in a suicide attack in Kabul after leaving a government-sponsored conference of the Ulema Council at which religious leaders from both Sunni and Shia sects condemned suicide attacks and the militants' war against the US-backed Afghan Government.
Isis claimed responsibility for the June attack, denouncing the meeting of "tyrant clerics" and their condemnation of suicide attacks, according to a website affiliated with the group.
The Afghan Ulema Council had issued an unprecedented religious edict earlier that day that said the insurgency in Afghanistan has no religious basis. It also declared that suicide attacks, often used by Taliban and Isis insurgents, are "haram," or forbidden by Islam.
Affiliates of Isis have repeatedly targeted mosques and sites of worship of Afghan Shia Muslims in recent years. Isis is a radical Sunni group, regards Shias as heretics.
This month, a deadly blast targeted a demonstration by hundreds of minority Shias in the capital. Afghan officials said several people were killed in the November 12 explosion near a high school and close to a gathering of people protesting against Taliban attacks on Shia areas in the Jaghuri and Malistan districts of eastern Ghazni province.