At least 137 people were killed and 357 wounded when suicide bombers, pretending to be disabled and hiding explosives under casts, attacked the mosques in Sanaa, according to Yemen's state-run Saba news agency.
Video distributed by Reuters showed people removing bodies from one of the mosques, where a carpeted floor was littered with debris.
If ISIS committed the attack, it would be not only a new challenge to the minority Houthis who took control of Sanaa weeks ago, but also a challenge to ISIS's rival, al Qaeda in the Arabian Peninsula, the Sunni extremists more associated with Yemen.
It would also illustrate a seemingly expanding focus for ISIS, which controls parts of Syria and Iraq and earlier this week claimed responsibility for Wednesday's killings of tourists at a museum in Tunisia.
Regardless, Friday's attacks marked one of the worst days of recent violence during a complicated struggle for control of Yemen, where Houthi rebels -- Shiites in a predominantly Sunni country -- already faced resistance from AQAP and from supporters of ousted President Abdu Rabu Mansour Hadi.
A written statement, purportedly from ISIS, claimed that ISIS executed Friday's attacks, calling them "a tip of an iceberg."
The statement, posted on a site that previously carried ISIS proclamations, said five suicide bombers targeted Houthis in Sanaa.
A separate audio message, also posted on ISIS-affiliated websites, claimed five ISIS suicide bombers killed dozens of "Houthi infidels." The voice is similar to one featured in Thursday's audio message in which ISIS claimed responsibility for Wednesday's attack at the Bardo Museum in Tunisia.
CNN cannot independently verify the legitimacy of Friday's statements.
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Among those killed in Sanaa was prominent Houthi religious leader Murtatha Al Mahathwari, Saba reported.
A separate explosion rocked a government compound in the Houthi stronghold city of Saada -- 180 kilometers (112 miles) northeast of Sanaa -- killing two people and seriously wounding a third, according to Abu Khalil Al Ameri, a local Houthi security official.
If ISIS executed Friday's bombings, it would be a "big deal indeed," in part because ISIS was thought to have only a fledgling presence in Yemen, CNN terrorism analyst Paul Cruickshank said.
"The dominant group there is al Qaeda in Yemen. ISIS and al Qaeda in Yemen can't stand each other," Cruickshank said. "But ... whoever is responsible, I think, (is) trying plunge the country into civil war."
That's a complication for a number of parties, including the United States, which until the Houthis took Sanaa had counted on the Yemeni government for support in its long-running battle against AQAP.
Blasts inside, outside mosques
In Friday's assaults at Al Badr mosque and Al Hashoosh mosque in Sanaa, the first blasts happened inside the buildings, followed two minutes later by explosions outside, perhaps to target those fleeing the preliminary blasts, two senior Houthi leaders in Sanaa said.
At Al Badr mosque, the outdoor explosion was another suicide bombing; at Al Hashoosh mosque, the exterior blast was a car bomb, the two leaders said.
"We check people and watch at times, but it's a mosque, and we can't check everyone who enters," said Ali Al Emad, a Houthi security worker at Al Hashoosh mosque.
The mosques serve members of the Zaidi sect of Shiite Islam, the sect to which the rebel Houthi militants belong.
Among the wounded are 40 people who are in critical condition; they will be sent to Jordan for treatment, reported Saba, which is controlled by the Houthis.
AQAP, which last year vowed to attack Houthi loyalists throughout Yemen, issued a statement saying it had nothing to do with Friday's bombings.