That sparked a panic, and people, including many women and children, began stampeding, they said.
Motaher al-Marouni, a senior health official in Sanaa, gave the death toll of 78 and said at least 13 were seriously hurt, according the rebels' Al-Masirah satellite TV channel.
Hamdan Bagheri, deputy director of the al-Thowra Hospital in Sanaa, said in televised comments that the tragedy took place around 8.20 p.m. and the facility received at least 73 injured people.
The rebels quickly sealed off the school where the event was held and barred people, including journalists, from approaching.
Yemen's capital has been under the control of the Iranian-backed Houthis since they descended from their northern stronghold in 2014 and removed the internationally recognized government.