Two car bombs have rocked the northern Lebanese city of Tripoli, killing at least 27 people. Health officials say more than 300 people were wounded.
The blasts occurred outside mosques in Tripoli as Friday prayers ended.
The Associated Press says one of the blasts exploded outside the Taqwa mosque, where Sheik Salem Rafei, a Salafi cleric opposed to Lebanon's militant Shiite Hezbollah group, usually prays. It was not clear whether he was inside. The second went off about five minutes later.
There was no immediate claim of responsibility for the attacks.
Tensions have been on the rise in Lebanon during the civil war in neighboring Syria, and especially in Tripoli, where Sunnis, who support the armed uprising in Syria, have clashed repeatedly with Alawites, who support President Bashar al-Assad.
The U.S. Embassy in Beirut condemned the bombings "in the strongest terms" and called all all parties to exercise calm and restraint.
Salman Shaikh, director of the Brookings Doha Center, warned via Twitter it is now only a matter of time before Sunni groups seek revenge, saying, "The cycle of sectarian tit-for-tat will only get worse."
An anti-Hezbollah Islamist group calling itself the Brigades of Aisha claimed responsibility last week for a car bombing in the suburb of Rweiss, south of Beirut, a stronghold of Lebanon's Shi'ite militant group Hezbollah. That blast killed 22 people and wounded more than 200.