"I saw tens of people laying down in blood around me and hundreds of people running away from the scene," said Fatima Faizi, an Afghan freelance journalist.
So far, 80 bodies and more than 260 wounded people were taken to hospitals in Kabul, according to Ismail Kawoosi, a spokesman for the Afghan Health Ministry.
Sayed Hamed, 30, attended the protest but left before the explosions. He was about two kilometers (1.2 miles) away.
"As I was watching (from my hotel) and some (people) were running toward the scene and some were crying coming from the scene," he wrote in an email to CNN. "It was a very sad situation, and everyone was trying to find their relatives or friends."
The attack, the worst in months in terms of casualties, drew attention to ISIS instead of the Taliban, which had been blamed for recent bombings.
Two ISIS fighters detonated their suicide belts among the protesters, according to ISIS' media wing, Amaq. A third attacker was killed by security forces before detonating his bomb, according to an Afghan security official speaking on condition of anonymity.
The White House condemned the "horrific attack," adding that it was made "all the more despicable by the fact that it targeted a peaceful demonstration."
The UK's Foreign and Commonwealth Office echoed the sentiment. FCO Minister Alok Sharma said "the UK remains steadfast and resolute in our long-term commitment to the Afghan people. We will continue to work with the Government of Afghanistan to help build a more stable and secure Afghanistan."
In a separate incident Sunday, a rocket landed in a kindergarten yard in the Macroryan district of Kabul, Sediq Sediqqi, Afghanistan Interior Ministry spokesman told CNN. Even though no casualties were reported, Afghanistan remained on high alert Sunday, Sediqqi said.
The jihadist terrorist group has been stepping up attacks worldwide -- and most recently in Afghanistan -- while losing territory in its self-proclaimed caliphate in Iraq and Syria.
The blast in Kabul on Saturday afternoon happened during a demonstration by members of the Hazara, a Shiite minority group, near the Afghan Parliament building and Kabul University. Declaring a national day of mourning Saturday evening, Afghanistan President Ashraf Ghani ordered flags in the nation to be flown at half-staff.
The protesters were demanding a planned power line be rerouted through their poverty-stricken Bamyan province to ensure electricity in the relatively isolated area west of Kabul.
Accounting for up to one-fifth of Afghanistan's population, Hazaras, a Persian-speaking people who mainly live in central Afghanistan, have long been branded outsiders for their Shia faith and far Asian features in the country dominated by followers of the Sunni branch of Islam, according to a 2008 National Georgraphic article
The Hazara in the past have demanded the government protect them from attacks blamed on the Sunni Taliban and ISIS.
On November 11, thousands of protesters marched through Kabul with coffins containing the decapitated bodies of seven Hazaras, four men, two women and one child. The protesters called for justice for the beheadings, chanting slogans seeking death for the Taliban and ISIS.
Saturday's attack is the latest in a rash of kidnappings and bombings in Kabul, which have heightened security fears in the nation's capital.
The Taliban have claimed responsibility for other attacks.
Three weeks ago, two Taliban suicide bombers killed 34 people
when they attacked a convoy of buses carrying newly graduated police officers in Kabul.
On June 20 in the Afghan capital, a suicide bomber killed
14 Nepali security contractors who worked for the Canadian embassy.
The Taliban claimed responsibility for that attack in a text message sent to media organizations.
U.S. and other diplomats were barred from traveling by road the short distance from the city's international airport to their diplomatic missions. Instead, they were ferried by helicopter.
Meanwhile, the 14-year war against the Taliban in the countryside is as bloody as ever. While the Taliban is the dominant insurgent force in the central Asia country, ISIS has been establishing a presence.
U.S. President Barack Obama announced on July 6 that he would slow the planned drawdown
of the 9.800 troops supporting the Afghan country because of the precarious security situation, including the emerging threat from ISIS.
In the last 18 months, 38 Americans, both civilian and military, have died in the country.
Obama noted the United States was no longer engaged in a major ground war as it was in 2009 when he took over with plans to end American involvement, but he said, "Afghanistan ... remains one of the poorest countries in the world. It is going to continue to take time for them to build up military capacity that we sometimes take for granted. And given the enormous challenges they face, the Afghan people will need the partnership of the world, led by the United States, for many years to come."
Other ISIS attacks in Afghanistan
ISIS attacks or ISIS-inspired attacks in Afghanistan include:
• April 18, 2015 - A suicide bomber on a motorbike blew himself up in front of a bank in Jalalabad, Afghanistan, killing at least 33 people and injuring more than 100 others. "On April 18, local media received text messages allegedly from Shahidullah Shahid, a key figure in the establishment of Wilayat Khorasan, claiming responsibility on behalf of ISIS for the attack," the Institute for the Study of War reported. Wilayat Khorasan is the ISIS affiliate in Afghanistan and Pakistan. The attack is believed to have been conducted by ISIS or one of its affiliates.
• January 13, 2016 - Three ISIS fighters launched an attack on the Pakistani consulate in Jalalabad. The first operative reportedly detonated his explosives while attacking the consulate's guards and a second bomber detonated his device inside the building. A local official said six people were killed, as well as the three assailants. The attack is believed to have been carried out by ISIS or one of its affiliates.
• June 5, 2016 - The Khorasan Province of ISIS claimed it had carried out the assassination of Afghan member of parliament Sher Wali Wardak. Wardak was killed by an IED while in his car in Kabul. The attack is believed to have been carried out by an ISIS affiliate.
• June 20, 2016 - The Khorasan province of ISIS claimed responsibility for a suicide attack on a bus carrying Nepalese security guards in Kabul. At least 16 people were killed in the attack. ISIS identified the bomber as Irfanullah Ahmed and published a photo of him. But the Afghan Taliban also claimed responsibility for the attack. The origins of the attack are unknown.