Twin explosions outside two mosques killed at least 27 people and wounded hundreds in apparently coordinated attacks in the northern Lebanese city of Tripoli on Friday, a senior health official and witnesses said.

The blasts, the biggest and deadliest in Tripoli since the end of Lebanon's 1975-90 civil war, struck as Friday prayers ended in the largely Sunni Muslim city. They came a week after a huge explosion killed at least 24 people in a stronghold in Beirut of the Shi'ite Muslim militant movement Hezbollah.

A recent resurgence of sectarian violence in Lebanon has been stoked by the war in adjacent Syria, where President Bashar al-Assad is fighting a largely Sunni-led rebellion and Hezbollah has sent fighters into combat on his side.

"I see seven bodies inside several burned cars," said a Reuters witness in Tripoli, speaking from the site of the first blast outside the Taqwa mosque that is frequented by hardline Sunni Islamists. At least 14 people were killed there.

The other deaths were reported from the second blast outside the al-Salam mosque. At least 358 people were wounded, Health Minister Ali Hassan Khalil told Reuters.

Ambulances rushed to the scene of the Tripoli blasts and heavy black smoke covered the skyline of the Mediterranean coastal city.

Television footage showed a large crater outside the Salam mosque, and a large blast zone of crushed and burning cars. People ran through the streets, some of them carrying the bloodied bodies of the wounded.

Two corpses could be seen on the ground and apartment blocks had their windows and balconies smashed.

Afterwards, angry gunmen took to the streets of Tripoli and fired in the air. Near the blast sites, angry men threw rocks at Lebanese soldiers examining the aftermath.

The southern suburb of Beirut dominated by Hezbollah has been hit by two car bombs in just over a month. Hezbollah leader Sayyed Hassan Nasrallah blamed radical Sunni Islamists.

Salem al-Rafei, chief cleric of the Taqwa mosque, is a staunch supporter of Syrian Sunni rebels as well as Lebanese Sunni militants who have joined the anti-Assad battle in Syria.