The young protesters of various ages chanted anti-Saudi slogans and carried banners in both Arabic and English. “Who allowed you to shed the blood of the children of Yemen?” one of the signs, addressed to Riyadh, read.

Some of the children held the postmortem photos of the schoolkids, who died in a Saudi-led airstrike that hit a packed school bus in the town of Dahyan in the northern Saada Province on Thursday. 51 people, including 40 children between 10 and 13 years of age, were killed and 79 others injured in the attack, according to Taha al-Mutawakil, the health minister for the Houthi authorities in Yemen.

“We, the children of Yemen, demonstrate in pain and compassion, in solidarity with our brothers and sisters, mothers, fathers in the hurt region of Saada, who suffered the worst harm at the hands of the attacker,” one young protester said, adding that the Saudis were “crazy” for targeting the kids.

The killing of children was condemned by numerous international organizations, with UN Secretary General António Guterres and the UN Security Council urging a credible and transparent investigation of the incident.

The Saudi-led coalition said that it targeted missile launchers used to attack the city of Jiza in southern Saudi Arabia and blamed the Houthi rebels for using children as a human shield.

Put under international pressure, it later promised to carry out an investigation of the airstrike. However, similar pleas were made to Riyadh after previous attacks, in which civilian lives were lost, but they brought no disciplinary action or change in the way the airstrikes are conducted.

Saudi Arabia and its allies intervened in Yemen in March 2015 in order to fight the Houthi rebels and help reinstate ousted President Mansour Hadi to power. Since then, more than 10,000 people have been killed and over 40,000 injured as of the end of 2017, according to the UN.

The majority of civilian casualties came from airstrikes, with the Saudi-led coalition being blamed by international human rights groups for indiscriminate bombings, targeting hospitals, markets and other infrastructure.

In early August, at least 26 people were killed and dozens injured in the bombing of a hospital in the Red Sea port of Hodeidah. An airstrike on a Yemeni wedding ceremony claimed 130 lives in September 2015, becoming one of the deadliest single attacks in the conflict so far. Over a hundred people were also killed last year when a funeral came under fire from Saudi planes.

After more than three years of the Saudi-led intervention, Yemen is facing a massive humanitarian crisis. More than 22.2 million people in the country are in need of assistance, with 60 percent of the population lacking food and more than half of Yemenis left without basic medical services, the UN estimates.