Afghanistan is observing a day of national mourning following the suicide bombing of a protest march in the capital, Kabul, that killed 80 people and wounded 230.
So-called Islamic State (IS) has said it was behind Saturday’s attack on members of the Shia Hazara minority.
In a televised address, President Ashraf Ghani vowed to take revenge against those responsible.
The UN mission in Afghanistan has described the attack as a “war crime”.
Tadamichi Yamamoto, head of the UN assistance mission, said the attackers had specifically targeted a large number of civilians.
On Saturday, the IS-linked Amaq news agency said two fighters had detonated explosive belts at a “gathering of Shia” in Deh Mazang square.
Correspondents say the statement suggests an intention to foment sectarian strife.
Thousands of members of the Hazara minority were protesting over a new power line, saying its route bypasses provinces where many of them live.
The Taliban condemned the attack.
IS has a presence in eastern Afghanistan but this is the first time it has admitted carrying out attacks in the capital.
An Afghan intelligence source told the BBC that an IS commander named Abo Ali had sent three jihadists from the Achen district of Nangarhar province to carry out the Kabul attack.
The interior ministry said only one attacker had successfully detonated his explosives. The belt of the second failed to explode and the third attacker was killed by security forces, the source said.
Declaring Sunday a day of national mourning, Mr Ghani said: “I promise you I will take revenge against the culprits.”
He had earlier issued a statement saying that peaceful protest was the right of every citizen and that “opportunist terrorists” had infiltrated the crowd.
The Hazaras – mostly Shia Muslims – live mainly in the centre of the country.
They have long complained of discrimination. During Taliban rule in the late 1990s, many of them fled to Pakistan, Iran and Tajikistan.
Who are the Hazaras?
- Of Mongolian and Central Asian descent
- Mainly practise Shia Islam, in predominantly Sunni Afghanistan and Pakistan
- Thought to be the third largest ethnic group in Afghanistan
- Estimates suggest they make up 15-20% of Afghanistan’s population, which is thought to be about 30 million
- At least 600,000 Hazaras live in Pakistan, most of them in Quetta
- Legend has it they are descendants of Genghis Khan and his soldiers, who invaded Afghanistan in the 13th Century