As reported by Censor.NET citing CNN, at least 29 were killed.
Officials fear the death toll from the attack at the Al Bilaili in Sanaa will rise because dozens of worshipers suffered severe injuries.
No one immediately claimed responsibility for the attack.
The attack is the sixth in three months in which a mosque has been targeted.
The early morning prayer service was meant to mark the beginning of Eid al-Adha, which commemorates the prophet Abraham's willingness to sacrifice his son for God. The holiday is one of two major ones in Islam.