At least two people have died and scores have been injured after a tiered seating structure collapsed at a partly built synagogue north of Jerusalem.
Police said about 650 ultra-Orthodox worshippers were in the synagogue at Givat Zeev, a Jewish settlement in the occupied West Bank, to mark a festival.
Footage showed the seating collapse and people falling on top of each other.
It comes weeks after 45 people died in a crush at an ultra-Orthodox festival on Mount Meron in northern Israel.
In Sunday's incident, initial reports from Israel's Magen David Adom ambulance service said medics "provided care to about 60 injured, including about 10 in critical and severe condition".
A later update said two people were confirmed dead and about 100 were injured.
Witnesses said the collapse happened during a ceremony to mark the beginning of the Jewish holiday of Shavuot. Video showed hundreds of worshippers dancing and singing just before the seating gave way.
Prime Minister Benjamin Netanyahu said he was praying for the injured.
The synagogue was described as partially constructed. Police and local officials said organisers had been warned that the building was unsafe and the event had taken place despite a lack of permits.
"We were called again to another event where there was negligence and a lack of responsibility. There will be arrests," Jerusalem district police chief Doron Turgeman told television reporters.
Israel's government watchdog has launched an inquiry into the deadly crush at Mount Meron - one of Israel's worst civilian disasters - saying it could have been prevented. All the victims were men or boys.