Suicide bombers have attacked two mosques in Yemen's capital, Sanaa, killing at least 126 people and wounding many others, reports say.
Worshippers were attending noon prayers at the Badr and al-Hashoosh mosques when at least four attackers struck.
The mosques are used mainly by supporters of the Zaidi Shia-led Houthi rebel movement, which controls Sanaa.
Islamic State (IS), which set up a branch in Yemen in November, said it was behind the attacks.
A statement from the group was published on Twitter accounts known as reliable sources for IS propaganda. If confirmed, the attacks would be the first carried out by IS in Yemen.
There are also severe tensions between the Houthi rebels and various powerful, armed elements in Yemen, including militants from al-Qaeda.
Blood 'running like river'
Witnesses said two suicide bombers attacked the Badr mosque, in the south of Sanaa.
One entered the building and detonated his explosive device among dozens of worshippers, the witnesses added. Survivors then sought to escape through the main gates, where the second bomber was waiting.
Al-Jazeera reported that the prominent Houthi cleric al-Murtada bin Zayd al-Mahatwari, the imam of the Badr mosque, was among those killed.
Two more bombers attacked the al-Hashoosh mosque, in the north of the capital, with one detonating explosives near the entrance and the other running into the mosque itself.
One worshipper said he was thrown about 2m (6ft) by the blast.
"The heads, legs and arms of the dead people were scattered on the floor of the mosque," Mohammed al-Ansi told Associated Press news agency, adding that "blood is running like a river".
Mr Ansi said that many of those who were not killed by the explosion were seriously injured by shattered glass that fell from the mosque's windows.
The rebel-run al-Masirah TV channel broadcast footage from inside the al-Hashoosh mosque showing volunteers using bloodied blankets to carry away victims. Bodies were also lined up in the prayer hall.
More than 260 people were said to have been wounded. Al-Masirah reported that hospitals in the city had made urgent appeals for blood donations.
Another suicide bomber targeted a mosque in the northern city of Saada - a Houthi stronghold - reports said, but only the attacker was killed.
The White House "strongly condemned" the attacks, but said it could not confirm that those behind them were affiliated with IS.
Yemen is the base of al-Qaeda in the Arabian Peninsula (AQAP), a powerful offshoot of the jihadist militant group that has carried out similar suicide attacks on Houthi supporters.
However, IS is also gaining ground in the country.
AQAP denied carrying out Friday's bombings, citing instructions from its leader Ayman al-Zawahri not to target mosques or markets.
The bombings come a day after deadly clashes in the southern city of Aden, between forces loyal to President Abdrabbuh Mansour Hadi and those supporting his predecessor, Ali Abdullah Saleh.
Mr Hadi has been based there since fleeing Sanaa last month after the rebels placed him under effective house arrest.
On Friday warplanes targeted the presidential palace in Aden for a second day but Mr Hadi's aides said he was unharmed.
There were also reports on Friday that al-Qaeda fighters had taken control of the southern city of al-Houta.
They reportedly took over barracks and government buildings and some security force officers are said to have been killed.
Mr Saleh was forced to hand over power to Mr Hadi in 2011 after mass protests, but has remained a power-broker. He is currently allied with the Houthis, against whom he fought wars when he was president.