A suicide bomber drove into a school in northern Yemen's al-Jawf province where members of a Shia rebel group had gathered for Friday prayers and killed 12 people, the defence ministry said.
A local official said several people were wounded in the attack in the province that borders Saudi Arabia.
Witnesses told the AFP news agency that a car rammed a school in the town of Hazm, which Shia Houthi [Zaidi] rebels have turned into a base.
"The school's entrance was completely destroyed," said the tribal chief who spoke to AFP on the condition of anonymity.
He said that shrapnel flew 500m from the site of the explosion "due to its intensity".
Hasan Bughadra, an activist from the tribes of al-Jawf, also said a number of rebels were among those killed.
"The toll is expected to rise especially among civilians because the school is located right in the town centre," he said.
Earlier on Friday, another suicide bomber targeted a Houthi protest march in the northern province of Saada, where the rebels have effectively carved out their own state within a state.
There were no casualties in the attack, the Houthis said in a statement.
"As the march was taking its first steps, the suicide bomber, who was wearing an explosive belt, tried to breach the security barriers and enter the square," read a statement from the Houthis' media office.
"But those responsible for protecting the protest stopped and searched him, which made him nervous and he lost control,
blowing himself up erroneously."
There was no claim of responsibility for either of Friday's attacks. Sunni fighters affiliated with al-Qaeda sometimes view Shias as heretics.
The Houthis have fought regularly with Sunni Salafis attending a religious college in Saada. They accuse Saudi Arabia of smuggling weapons to the Salafis because the two follow a similar creed.
Western and Gulf Arab states have also watched with alarm as al-Qaeda-linked fighters have exploited political instability in Yemen to gain a foothold there.
On Monday, a suicide bomber dressed in army uniform blew himself up in the middle of a military parade rehearsal in the capital Sanaa, killing more than 90 soldiers and wounding more than 200 people.
An al-Qaeda affiliate claimed responsibility for that attack.