About 100 young men, mostly Islamists, blocked the northern and southern roads into Tripoli on Saturday, demanding the release of a fellow resident accused of terrorism, an AFP correspondent said.
The protesters set up camp at the southern entrance of Tripoli, the largest city in northern Lebanon.
Black flags bearing the profession of Islam, "God is Great", were planted alongside the Syrian flag of independence, a symbol of revolt in the neighbouring country.
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"We will not leave until my brother is released," Nizar al-Mawlawi, whose 27-year-old brother Shadi was arrested by Lebanese security forces on Saturday.
According to a statement from the Lebanese security services, Shadi al-Mawlawi was arrested as part of an "investigation into his ties to a terrorist organisation," without going into details.
Syrian authorities have repeatedly charged that arms and fighters are being smuggled in from Lebanon to help the rebels fighting to overthrow President Bashar al-Assad.
Lebanon is divided between the opposition, backed by Washington and hostile to the Syrian regime, and the camp of the Shiite group Hezbollah, which dominates the government and is supported by Damascus and Tehran.