At least two or three armoured personnel carriers were deployed around the prime minister's office. A man over a megaphone chanted: "Declare it a revolution!"
The protest is expected to be the largest of the demonstrations that began last week over garbage piling up in the streets of Beirut, following the closure of a main landfill. But the government's failure to resolve the crisis has evolved into wider protests against a political class that has dominated Lebanon since the end of the country's civil war in 1990.
There were concerns that the protest would also descend into clashes. To avoid friction with security forces, organisers of the protest shifted the location from Riyad Solh square opposite the government building known as the Grand Serail to Martyr's Square, a major square few hundred meters away.
The government said a joint security-military operations room was set up to prevent chaos.
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The London-based rights group Amnesty International called on Lebanese authorities today to investigate allegations that security forces have used excessive force to disperse rallies.
Amnesty said security forces fired live rounds, used rubber bullets and hurled stones or beat protesters, leaving 59 people hospitalized. It called on security forces to refrain from using "unnecessary or excessive" force during the protest.
"Everyone in Lebanon has the right to peaceful assembly. Lebanese officials must uphold this right and send a clear message to security personnel that such attacks against peaceful protesters will not be tolerated," said Lama Fakih, senior crisis adviser at Amnesty International.