At least 80 people were killed and more than 230 wounded Saturday when attackers detonated explosives amid a huge crowd of peaceful protesters in the Afghan capital, most of them from the country’s Shiite ethnic Hazara minority, Afghan officials said.

Spokesmen for the Islamic State quickly claimed responsibility for the attack at a traffic circle jammed with demonstrators, according to Afghan media. The group’s media office said two Islamic State fighters detonated suicide belts among the crowd, in two separate bombings.

The death toll was the highest for any terrorist attack in the capital after more than a decade of fighting between Taliban militants and Afghan and NATO forces. If indeed carried out by the Islamic State, known as Daesh in Afghanistan, it would be the first major urban attack in the country by the radical Sunni terrorist group and could signal its first deliberate effort to target Afghanistan’s Shiite minority, which it views as infidels.

Hundreds of Hazaras have reportedly fought alongside President Bashar al-Assad’s troops in Syria against Sunni groups, including the Islamic State, in recent years, making Hazaras a likely target for the extremist group’s loyalists back in Afghanistan.

Until now, the Middle East-based Islamic State has been active mainly in eastern Afghanistan, near the Pakistani border. On Saturday, Afghan intelligence officials said the group had sent several fighters from that region to stage the attack in Kabul, the BBC reported. The domestic Taliban insurgency has carried out numerous bombings and other attacks in the capital over the past several years.

Until Saturday’s blast, the deadliest single attack in Kabul had been in December 2011, when more than 70 people were killed in a suicide bombing near a mosque where Shiite mourners were observing Ashura, a day that marks the killing of the prophet Muhammad’s grandson Imam Hussein and his followers in A.D. 680. A Pakistani militant group claimed responsibility for the attack. Bombings also took place in two other Afghan cities that day.

On Saturday, the Taliban denied any connection to the latest attack. A spokesman for the group, which is also Sunni Muslim, called the bombing “an ominous plot aimed at creating discord among the nation.” During the late 1990s, when the Taliban regime held power in Kabul and most of the country, it banned observing Shiite religious holidays in public.

Saturday’s bombing took place in West Kabul near a police building, the city’s zoo, the national university and the national parliament. Hazara protesters had marched and gathered there in the latest of several large peaceful protests against plans for a power line from Central Asia that would bypass two impoverished provinces in the Hazara heartland, demanding that the government undertake a large power project to bring electricity to Bamian province, a Hazara-majority region in north-central Afghanistan.

Officials from the rights group Amnesty International said the “horrific attack” was a reminder that the conflict in Afghanistan “is not winding down, as some think, but escalating, with consequences for the human rights situation in the country that should alarm us all.”

The White House issued a statement condemning the “heinous” attack, saying it “was made all the more despicable by the fact that it targeted a peaceful demonstration.”

The Hazara demonstration, which followed several others in May, had been announced in advance, and its route and location were well-known. As in the previous protests, the government had blocked major routes from West Kabul to the presidential palace and downtown, using shipping containers as well as lines of police.

As a result of the road closures, officials said, it was difficult to transport victims to major hospitals, and smaller clinics and health facilities near the blast site were overwhelmed. Among the wounded was a member of parliament, Ahmad Behzad, witnesses said.

Despite the devastating attack, some protesters regrouped and gathered near the site later in the day, vowing to continue their protest until Afghan President Ashraf Ghani accepts their demands. In one area, angry demonstrators chanted slogans against the government and threw stones at security forces.

Both Ghani and the government’s chief executive officer, Abdullah Abdullah, issued statements condemning the attack. Ghani, speaking at a public gathering, declared Sunday a day of national mourning. He also said security forces had shot dead an additional suspected suicide bomber at the scene.

Afghanistan’s Hazara minority, historically an oppressed group that suffered at the hands of ethnic Pashtun rulers as well as the Taliban, has been emerging as an ambitious political force in recent years under democratic rule. Last year, Hazara demonstrators converged on Kabul to protest the terrorist slaughter of a group of Hazara civilians, in the largest-ever demonstration in the Afghan capital.

Constable reported from Chincoteague, Va.