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After the blast people ran through the streets trying to carry wounded people to safety.
At least 27 people have been killed and more than 350 injured by two blasts in Lebanon's northern city of Tripoli, officials say.
As Friday prayers ended, a blast hit the al-Taqwa mosque, usually attended by prominent Sunni cleric Sheikh Salem Rafii. He was unharmed.
A second blast five minutes later hit the al-Salam mosque.
War in neighbouring Syria has raised sectarian tensions between the city's Sunni Muslim and Alawite communities.
'Like an earthquake'
Sheikh Salem Rafii is one of the most prominent Sunni leaders in Lebanon, BBC Arabic reports from Beirut, and is believed to be a possible target of the attack.
He is opposed to Lebanon's militant Shia Hezbollah group and has previously urged the country's young men to join opposition fighters in Syria.
It is not clear whether he was at the mosque at the time of the attack, although some reports say he was giving a sermon.
The BBC's Yolande Knell in Beirut says the cleric is expected to issue a statement after meeting the Muslim Clerics' Council, the umbrella body for Lebanese Sunni leaders.
Ambulances rushed to the aftermath of the blasts and heavy black smoke covered the sky.
"It was as if there was an earthquake, the whole city seemed to be shaking," a local resident told Lebanon's Daily Star newspaper.
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.
The Associated Press reported that the preacher at the al-Salam mosque - the site of the second explosion - is also an opponent of the Syrian government and its Lebanese ally, Hezbollah.
No group has taken responsibility for the latest attacks.
Tripoli, a city of nearly 200,000 people and Lebanon's second largest, is one of the country's most volatile sectarian fault lines, with a small Alawite population living in the midst of a Sunni majority.
The Alawite community tend to support Syrian President Bashar al-Assad, with Sunnis mostly backing the rebels fighting him.
The bombs come a week after a massive car bomb rocked a Shia district of Beirut, leaving dozens dead. The area hit contained Hezbollah strongholds.
Outgoing Lebanese Prime Minister, Najib Mikati, condemned the latest deadly explosions, saying they aimed to "incite strife" and "drag Tripoli and it sons into reactionary moves".
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