A medical official has said at least 142 people were killed when suicide bombers blew themselves up in two mosques in the Yemeni capital Sanaa today.
Four bombers wearing explosive belts targeted worshippers in and outside the crowded mosques.
The attacks happened a day after an unidentified warplane attacked the presidential palace in the southern city of Aden.
At least 260 people were injured in the blasts.
The mosques are known to be used mainly by supporters of the Shia Muslim Houthi group, which has seized control of the government.
Yemen is torn by a power struggle between the Iranian-backed Houthis in the north and the UN-recognised President Abd-Rabbu Mansour Hadi.
The self-styled Islamic State group claimed responsibility for the attacks.
In an online statement, the previously unheard of Sanaa branch of IS warned that the bombings were "just the tip of the iceberg".
Hospitals in Sanaa appealed for blood donors to help treat the large number of casualties.
A third suicide bomber tried to blow himself up in one of the main mosques in the northern Houthi stronghold of Sadaa province but the bomb went off prematurely, killing the bomber only, a security source said.
Tensions have risen since Mr Hadi fled to Aden in February after escaping a month of house arrest in Sanaa by Houthi forces in control of the capital.
Mr Hadi has been trying to consolidate his hold over Aden, the better to mount a challenge to the Houthis' ambitions to control the whole country.