23 August 2013 Last updated at 08:18 ET
After the blast people ran through the streets trying to carry wounded people to safety.
The Lebanese Red Cross says more than 20 people have been killed by two blasts in the northern city of Tripoli.
As Friday prayers ended, a blast hit the al-Taqwa mosque, usually attended by prominent Sunni cleric Sheikh Salem Rafii. His fate is not clear.
A second blast five minutes later hit the city's al-Salam mosque.
War in neighbouring Syria has raised sectarian tensions in Tripoli between the city's Sunni Muslim and Alawite communities.
Sheikh Salem Rafii is one of the most prominent Sunni leaders in Lebanon, BBC Arabic reports from Beirut. He is opposed to Lebanon's militant Shia Hezbollah group.
It was not clear whether he was at the mosque at the time of the attack. The BBC's Yolande Knell in Beirut says it is not yet known what has happened to him.
Ambulances rushed to the aftermath of the blasts and heavy black smoke covered the sky.
Television pictures showed damaged cars on fire, with their windows smashed, and people running through the streets trying to carry wounded people to safety.
Bodies could be seen on the ground and windows were broken on surrounding apartment blocks.
Tripoli, a city of nearly 200,000 people and Lebanon's second largest, is one of the country's most volatile sectarian faultlines, with a small Alawite community living in the midst of a Sunni majority.
The bombs come a week after a massive car bomb rocked a Shia district of Beirut, leaving dozens dead.
The Lebanese Red Cross says more than 20 people have been killed by two blasts in the northern city of Tripoli.
As Friday prayers ended, a blast hit the al-Taqwa mosque, usually attended by prominent Sunni cleric Sheikh Salem Rafii. His fate is not clear.
A second blast five minutes later hit the city's al-Salam mosque.
War in neighbouring Syria has raised sectarian tensions in Tripoli between the city's Sunni Muslim and Alawite communities.
Sheikh Salem Rafii is one of the most prominent Sunni leaders in Lebanon, BBC Arabic reports from Beirut. He is opposed to Lebanon's militant Shia Hezbollah group.
It was not clear whether he was at the mosque at the time of the attack. The BBC's Yolande Knell in Beirut says it is not yet known what has happened to him.
Ambulances rushed to the aftermath of the blasts and heavy black smoke covered the sky.
Television pictures showed damaged cars on fire, with their windows smashed, and people running through the streets trying to carry wounded people to safety.
Bodies could be seen on the ground and windows were broken on surrounding apartment blocks.
Tripoli, a city of nearly 200,000 people and Lebanon's second largest, is one of the country's most volatile sectarian faultlines, with a small Alawite community living in the midst of a Sunni majority.
The bombs come a week after a massive car bomb rocked a Shia district of Beirut, leaving dozens dead.