Franz Rauchenstein, the head of the Red Cross delegation in Yemen, suggested that the death toll could be higher after visiting the site of the attack, saying relatively few detainees survived. A Red Cross statement said the detention center held around 170 detainees. It said 40 of those were being treated for injuries and the rest were presumed dead.
"Witnessing this massive damage, seeing the bodies lying among the rubble, was a real shock. Anger and sadness were natural reactions," Rauchenstein said.
The attack was the deadliest so far this year by the coalition, according to the Yemen Data Project, a database tracking the war. The coalition has faced international criticism for airstrikes that have hit schools, hospitals and wedding parties, killing thousands of Yemeni civilians.
Saudi Arabia intervened on behalf of the internationally recognized Yemini government in March 2015, after the Iran-backed Houthis took the capital. The conflict has claimed tens of thousands of lives, thrust millions to the brink of famine and spawned the world's worst humanitarian crisis.
The attack comes as the Saudi-led coalition's partners — chiefly the United Arab Emirates and an array of Yemeni militias — are increasingly at odds over the war's aims. The past weeks have seen heavy fighting in Yemen's south between Saudi-backed and Emirati-backed forces.
Yemeni officials said Sunday's strikes targeted a college in the city of Dhamar, which the Houthi rebels were using as a detention center. The coalition denied it had struck a detention center, saying it had targeted a military site used by the rebels to restore drones and missiles.
"We were sleeping and around midnight, there were maybe three, or four, or six strikes. They were targeting the jail, I really don't know the strike numbers," wounded detainee Nazem Saleh said while on a stretcher in a local hospital. He said the Red Cross had visited the center two times before the airstrike.
A line of over a dozen white body bags were laid out in the rubble beside flattened buildings and crushed cars, while rescue workers dug through the debris.
"We have seen now under the ruble that there are still many, many dead bodies that its very, very difficult to extract," said Rauchenstein.
The U.N. human rights office for Yemen said 52 detainees were among the dead, and at least 68 detainees were still missing.
The Red Cross, which inspects detention centers as part of its global mission, said it had visited detainees at the site in the past.
Former detainees said the Houthis had previously used the site to store and repair weapons.
Youssef al-Hadhri, a spokesman for the Houthi-run Health Ministry, said at least seven airstrikes hit three buildings in the complex overnight.
The rebels' Health Ministry said in a statement that more than 60 people were killed in Sunday's airstrikes and another 50 wounded. Later in the day, health officials said the death toll climbed to 65. The officials spoke on condition of anonymity because they were not authorized to brief media.
The Saudi-led coalition said it had hit a military facility "in accordance with international humanitarian law," and that "all precautionary measures were taken to protect civilians."
Col. Turki al-Maliki, a spokesman for the coalition, was quoted by the Saudi-owned Al Arabiya TV as denying the target was a prison.
Local residents said family members arrested for being critical of the Houthis were imprisoned in the detention center. They said at least seven airstrikes hit the area.
Omat al-Salam al-Haj, a mother of a detainee, said the center housed anti-Houthi political detainees who were rounded up over suspicions of cooperating with the coalition.
Former detainee Mansour al-Zelai said the Houthis were restoring weapons in and close to the detention center.
Houthi rebels have been using scores of sites as detention centers, including schools, mosques, and houses, filling them with thousands of political detainees to use later in prisoner-swap deals.
The Associated Press documented that many of these sites were rife with torture and abuses including Dhamar's community college.
Former detainees recalled torture and abuses inside the detention center, which came under a series of airstrikes before.
Rights groups have also previously documented that Houthis place civilian detainees in detention centers as human shields by placing them next to army barracks, under constant threat of airstrikes.
In October 2016, an airstrike by the Saudi-led coalition hit a prison complex in the Red Sea port of Hodeida, killing at least 58 people, mostly prisoners. At the time, the coalition said the prison complex was used as a command center for Houthis.
On Sunday, Sweden's foreign minister, Margot Wallstrom, met with Jordanian Foreign Minister Ayman al-Safadi in Amman to discuss her efforts to relaunch negotiations after years of stalemate between Yemen's warring sides.
"Yemen has been at the center of my attention," she said in a statement.
Wallstrom also met with Yemeni Prime Minister Maeen Abdulmalik Saeed and other government officials in Saudi Arabia's capital, Riyadh, according the official Yemeni news agency SABA.
Airstrikes have also punctuated the infighting between erstwhile coalition allies in southern Yemen.
On Thursday, Emirati jets bombed convoys of Saudi-backed government forces, killing scores in series of airstrikes to prevent them from retaking the interim capital, Aden, from militias backed by the UAE.
The Emirati strikes sparked popular anger in Yemen against the UAE. Activists launched an online petition collecting signatures to "kick Emiratis out of Yemen" and members of the Yemeni government issued a statement demanding the president end the UAE's role in Yemen.
On Sunday, UAE Minister of State for Foreign Affairs Anwar Gargash posted a reminder on Twitter that the coalition's goal is to "confront the challenge of the Houthi coup."