Suicide bombers targeted two Shi'ite mosques in Yemeni capital Sanaa, killing 55 and injuring hundreds, said medical authorities.
Three suicide bombers targeted mosques frequented by Shia Houthi militants, who seized control of the capital in September.
The bombers attacked the Badr and al-Hashoosh mosques during Friday midday prayers, traditionally when the mosques are busiest.
One witness at the al-Hashoosh mosque described the scene: "The heads, legs and arms of the dead people were scattered on the floor of the mosque," Mohammed al-Ansi told The Associated Press, adding, "blood is running like a river."
A local Houthi television station made an urgent appeal for blood donations, reports AP.
Children are reported to be among those killed.
No groups have claimed responsibility for the attack, but Al Qaeda vowed to wage war against the Houthi after they seized Sanaa, and moved into Sunni controlled regions.