At least 137 people have been killed and 345 wounded in suicide attacks on mosques in Yemen, according to a Shiite TV channel.
Four suicide bombers hit a pair of mosques controlled by Shiite rebels in the capital Sanaa, unleashing blasts through crowds of worshippers, according to medical officials.
The bombers attacked the Badr and al-Hashoosh mosques, located across town from each other, during midday Friday prayers, the most crowded time of the week, according to the state news agency.
While both mosques are controlled by Shiite rebels, known as Houthis, they are frequented by Sunni worshippers as well.
A group claiming to be a Yemeni branch of the Islamic State group said it carried out the bombings.
The group posted an online statement saying that five suicide bombers carried out what it described as a "blessed operation" against the "dens of the Shiites".
The claim offered no proof of an IS role. It was posted on the same website on which the IS affiliate in Libya claimed Wednesday's attack on a museum in Tunisia.
The Yemeni rebel-owned Al-Masirah TV channel said hospitals were urging citizens to donate blood. It also reported that a fifth suicide bomb attack on another mosque was foiled in the northern city of Saada, a Houthi stronghold.
Two suicide bombers attacked the Badr mosque. The first was caught by militia guards searching worshippers at the entrance and detonated his device at the outside gates. Amid the ensuing panic, a second bomber entered the mosque and blow himself up amid the crowds, according to the official news agency SABA.
Survivors compared the explosions to an earthquake, and said some of those who survived the original blasts were then injured by shattered glass falling from the mosque's large hanging chandeliers.
Another pair of suicide bombers attacked the al-Hashoosh mosque, according to the state news agency. One witness from that attack said he was thrown two metres by one of the blasts.
"The heads, legs and arms of the dead people were scattered on the floor of the mosque," Mohammed al-Ansi said, adding: "Blood was running like a river."
The television channel aired footage from inside the al-Hashoosh mosque, where screaming volunteers were using blankets to carry away victims. The dead included a small child, and corpses were lined up on the mosque floor and carried away in pick-up trucks.
A prominent Shiite cleric, al-Murtada al-Mansouri, and two senior Houthi leaders were among the dead, the TV channel reported.
If the IS claim is true, the bombing would be the first major attack by IS supporters in Yemen and an ominous sign that the influence of the group that holds much of Iraq and Syria has spread to this chaotic nation, where a powerful wing of al Qaida already operates.
The Houthis have controlled the capital since September and have been locked in battle with Sunni al Qaida fighters in various parts of the country. An official with al Qaida in Yemen said his group was not behind Friday's attack.
US officials said there was no indication that IS had any operational link to the bombings.
White House spokesman Josh Earnest said the US is still investigating the claim of responsibility by the Yemeni branch of Islamic State. He said the US is looking to see whether IS has a command-and-control structure that would enable it to co-ordinate the attack.
Mr Earnest said IS often claims attacks purely for propaganda value, but added that the attacks show everyone in the region - including Muslims - is in danger from IS.
He said there was also no clear evidence backing up the IS claim of responsibility for the attack in Tunisia this week.