The protests marked the sixth anniversary of the Cedar Revolution – a movement against Syrian influence that followed the February 2005 assassination of Prime Minister Rafik Hariri.
Highways leading into Beirut were blocked with traffic as opposition supporters carried national flags and banners of pro-Western political parties into the city's central Martyrs Square.
Security forces were deployed across the country.
The protests follow a drawn-out political crisis during which Hezbollah and its allies toppled the government of Saad Hariri – Rafiq’s son - over a UN-backed tribunal that would reportedly implicate members of Hezbollah in Rafiq Hariri's assassination.
The Hezbollah-supported Najib Mikati became prime minister and has been seeking to form a government for nearly two months.
The Hariri-led opposition said it will not recognise Mikati's government, which it calls "Hezbollah's cabinet".