At least two worshippers were killed and more than 150 injured on Sunday when a grandstand collapsed in a synagogue under construction in a Jewish settlement in the occupied West Bank, Israel's national ambulance service said.
The grandstand was packed with ultra-Orthodox worshippers and collapsed during prayers at the beginning of a major Jewish holiday.
A spokesman for Magen David Adom ambulance service said that paramedics had treated over 157 people for injuries and pronounced two dead; a man in his 50s and a 12-year-old boy.
The accident in Givat Zeev, just north of Jerusalem, raised further questions about safety measures at large ultra-Orthodox events, two weeks after 45 pilgrims were crushed to death in a stampede at the burial site of a Jewish sage in northern Israel.
A police spokesman said 650 worshippers were at the Givat worship site for the start of the Jewish holiday of Shavuot.
The event was held in a partially constructed synagogue. The local mayor and senior fire brigade and police officers said the event had gone ahead despite the lack of a permit and official warnings that the building zone was unsafe.
Security footage broadcast on local TV showed the crowded grandstand collapsing and worshippers falling on top of each other.
Israeli authorities traded blame.
The mayor of Givat Zeev said the building was unfinished and dangerous, and that the police had ignored previous calls to take action. Jerusalem police chief Doron Turgeman said the disaster was a case of "negligence" and that there would likely be arrests.
"We were called again to another event where there was negligence and a lack of responsibility. There will be arrests," Mr Turgeman said at the scene.
Deddi Simhi, head of the Israel Fire and Rescue service, told Israel's Channel 12 that "this building is not finished. It doesn't even have a permit for occupancy, and therefore let alone holding events in it."
Television footage from the scene showed the five-story building was incomplete, with exposed concrete, rebar, and wooden boards, and plastic sheeting as windows. A sign in Hebrew pasted to a wall of the building warned that "for safety reasons entrance to the site is forbidden".
Defense Minister Benny Gantz wrote on Twitter that "my heart is with the victims of the disaster in Givat Zeev."