Yemen ? A bomb rigged to motorcycle blew up amid a crowd of worshippers leaving Friday prayers at a mosque in a rebel stronghold of northern Yemen, killing at least 18 people and wounding about four dozen, officials said.

The attack occurred in Saada, a city in a mountainous area on the border with Saudi Arabia where a rebellion by Shiite Muslims of the al-Zaydi sect erupted in 2004. Thousands have died in violence between the rebels and the government of this predominantly Sunni country.

Both sides blamed each other for the attack in Saada, where officials said most of the 18 dead and approximately 45 injured were worshippers filing out of the Bin Salman mosque.