Twin explosions tore through a demonstration by members of Afghanistan's mainly Shi'ite Hazara minority in Kabul on Saturday, killing at least 80 people and wounding more than 230 in a suicide attack claimed by Islamic State.
Graphic television footage from the site of the attack showed many dead bodies lying on the bloodied road, close to where thousands of Hazara had been demonstrating against the route of a planned multi-million-dollar power line.
"Two fighters from Islamic State detonated explosive belts at a gathering of Shi'ites in the city of Kabul in Afghanistan," said a brief statement on the group's Amaq news agency.
If confirmed as the work of Islamic State, the attack, among the most deadly since the US-led campaign to oust the Taliban in 2001, would represent a major escalation for a group hitherto largely confined to the eastern province of Nangarhar.
The explicit reference to the Hazara's Shi'ite religious affiliation also marked a menacing departure for Afghanistan, where the bloody sectarian rivalry between Sunni and Shi'ite Muslims typical of Iraq has been relatively rare, despite decades of war. Islamic State is an ultra hardline Sunni group.
Officials in Afghanistan's main intelligence agency, the National Directorate for Security (NDS), said the attack was planned by an individual named Abu Ali, an Islamic State militant they said was based in Achin district in Nangarhar.
They said three bombers were involved in the attack.
The Persian-speaking Hazara, estimated to make up about 9 percent of the population, are Afghanistan's third-largest minority but they have long suffered discrimination, and thousands were killed during the period of Taliban rule.
"We were holding a peaceful demonstration when I heard a bang and then everyone was escaping and yelling," said Sabira Jan, a protester who witnessed the attack and saw bloodied bodies strewn across the ground. "There was no one to help."
The Taliban, a fierce, albeit Sunni enemy of Islamic State, denied any involvement and said in a statement posted on its website that the attack was "a plot to ignite civil war".
The attack succeeded despite tight security which saw much of Kabul city center sealed off before the demonstration, with stacks of shipping containers and other obstacles and helicopters patrolling overhead.
An Interior Ministry statement said 80 people had been killed and 231 wounded, with local hospitals straining to cope with those being brought in.
The worst previous attack against the Hazara was in December 2011, when more than 55 people were killed in Kabul during the Shi'ite festival of Ashura. That attack was claimed by a Pakistani Sunni militant group called Lashkar-e-Jhangvi.
President Ashraf Ghani declared a national day of mourning and vowed revenge, while the top UN official in Afghanistan, Tadamichi Yamamoto, condemned the attack as a war crime.
The United States and Russia condemned the attack and renewed pledges of security assistance to Kabul.
"We remain committed to work jointly with the Afghan security forces and countries in the region to confront the forces that threaten Afghanistan’s security, stability, and prosperity," the White House said in a statement.
Russian President Vladimir Putin reiterated his "readiness to continue the most active cooperation with … Afghanistan in fighting all forms of terrorism", Russian news agencies quoted a Kremlin statement as saying.
Saturday's demonstrators had been demanding that a 500 kV transmission line from Turkmenistan to Kabul be re-routed through two provinces with large Hazara populations, saying they feared being shut out of the project.
The government said the project guaranteed ample power to the provinces, Bamyan and Wardak, which lie west of Kabul, and that altering the planned route would delay it by years and cost millions of dollars. But the resentment felt by many Hazaras runs deeper than simple questions of energy supply.
In November, thousands of Hazara marched through Kabul to protest at government inaction after seven members of their community were beheaded by Islamist militants, and several protesters tried to force their way into the presidential palace.
The protests by a group whose leaders include members of the national unity government have put pressure on Ghani, who has faced growing opposition from both inside and outside the government.
They also risk exacerbating ethnic tensions with other groups and provinces the government says would have to wait up to three years for power if the route were changed.
The transmission line, intended to provide secure electricity to 10 provinces, is part of the so-called TUTAP project backed by the Asia Development Bank, linking energy-rich states of Central Asia with Afghanistan and Pakistan.