Afghanistan is marking a day of mourning on July 24 for more than 80 people who were killed in a suicide bombing that targeted peaceful protesters in Kabul on July 23.
Afghan President Ashraf Ghani has vowed to take "revenge" against those responsible for a suicide attack in Kabul that has killed at least 80 people, the deadliest attack to hit the Afghan capital since the U.S.-led invasion in 2001.
The Islamic State (IS) extremist group claimed responsibility for the twin suicide bombings, which the Interior Ministry said killed at least 80 people and wounded 231, many of them in serious condition.
The suicide attack at Kabul's Dehmazang Square targeted mostly members of the Hazara minority, thousands of whom had gathered to protest a power line.
If IS’s claims prove to be true, the bombings would mark the first time the militants have launched an attack in Kabul. IS fighters have seized pockets of territory along Afghanistan's eastern border with Pakistan, mostly in Nangarhar Province, in the past year.
In a live television address on July 23, Ghani said "I promise you I will take revenge against the culprits."
"I have ordered the attorney-general to set up a commission to investigate this incident," he said, adding that July 24 would be a national day of mourning.
Ghani’s spokesman said the government had received information that an attack on the demonstration could take place and had warned the organizers.
"We had intelligence over recent days and it was shared with the demonstration organisers. We shared our concerns because we knew that terrorists wanted to bring sectarianism to our community," presidential spokesman Haroon Chakhansuri told the AP.
The Islamic State group's Amaq news agency reported that two IS fighters detonated explosive belts at the peaceful march, which was attended by an estimated 10,000 people.
Kabul hospitals were overwhelmed, with reports emerging of blood shortages and urgent appeals for blood donors circulating on social media.
"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. "I saw many people were killed and most of them were covered with blood. There was nobody to help the victims. Policemen were looking at us and after that I heard gunshots. Then I don't know what happened."
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.
"The horrific attack on a group of peaceful protestors in Kabul demonstrates the utter disregard that armed groups have for human life," Amnesty International said in a statement.
"Such attacks are a reminder that the conflict in Afghanistan is not winding down, as some believe, but escalating, with consequences for the human rights situation in the country that should alarm us all."
Earlier, Ghani said in a statement that he was "deeply saddened" by the attack, adding that the casualties included security forces.
"Holding protests is the right of every citizen of Afghanistan and the government puts all efforts to provide security for the protesters, but terrorists entered the protests, and carried out explosions that martyred and wounded a number of citizens including members of security forces," the statement said.
Afghan Chief Executive Abdullah Abdullah condemned the "terrorist attack."
Gruesome photos circulating on social media showed horrific scenes with scores of people wounded in the square where the protesters had gathered.
Taliban militants denied involvement in the attack. "We want to make it clear," Taliban spokesman Zabihullah Mujahid said in an e-mail to media outlets. "Our mujahedin had no hand in the attack."
The local IS affiliate in Afghanistan is mainly made up of former Taliban fighters disillusioned by the group's failure to overthrow the government despite a 15-year insurgency.
WATCH: Before Bombing, Hazara Protest Over Power Line Route
The Hazara protesters were demanding that the 500-kilovolt transmission line from Turkmenistan to Kabul be rerouted through the central province of Bamiyan, which has a large Hazara population.
The government says this plan would cost millions and delay the project by years.
Demonstrators gathered near Kabul University, several kilometers from the main government area, waving Afghan flags and chanting slogans like "Justice! Justice!" and "Death to discrimination!”
The original plan was for the power line to run through the Bamiyan Province, where most of the country’s Hazara live.
The government says the new route, through the Salang Pass north of Kabul, would save millions of dollars in costs and expedite the project.
The Hazara are a Dari-speaking, Shi'ite community that has long been persecuted in Afghanistan.
They are considered the poorest of the ethnic groups and often complain of discrimination.
Only between 30 and 40 percent of Afghans are connected to the electric grid.