Dozens of protesters broke into the foreign ministry where they burnt a portrait of President Michel Aoun, a representative for many of a political class that has ruled Lebanon for decades and that they say is to blame for its current mess.
"We are staying here. We call on the Lebanese people to occupy all the ministries," a demonstrator said by megaphone.
About 10,000 people gathered in Martyrs' Square, some throwing stones. Police fired tear gas when some protesters tried to break through the barrier blocking a street leading to parliament, a Reuters journalist said.
Police confirmed shots and rubber bullets had been fired. It was not immediately clear who fired the shots.
Riot police shot dozens of teargas canisters at protesters, who hit back with firecrackers and stones.
TV footage showed protesters also breaking into the energy and economy ministries.
They chanted "the people want the fall of the regime", reprising a popular chant from the Arab Spring uprisings of 2011. They held posters saying "Leave, you are all killers".
The US Embassy in Beirut said the US government supported the demonstrators' right to peaceful protest and urged all involved to refrain from violence.
The embassy also said in a tweet that the Lebanese people "deserved leaders who listen to them and change course to respond to popular demands for transparency and accountability".
Prime Minister Hassan Diab said the only way out was early parliamentary elections.