Riot police fired tear gas and water cannon, chasing protesters armed with tree branches and signposts in a commercial district near Lebanon's parliament.
Crowds spilled on to the streets this week after a lull in largely peaceful protests which broke out in October. They are furious at a ruling elite that has steered the country towards its worst economic crisis in decades.
Police beatings and arrests in recent days have alarmed human rights groups and sparked fears among activists of a move to crush the dissent.
Rubber bullets fired
Smoke billowed out of tear gas canisters encircling protesters as ambulances sped through the streets of the capital. Witnesses said security forces also fired rubber bullets.
More than 100 injured, 15 detained
The Lebanese Red Cross said more than 100 people had been treated for injuries, with at least 65 others taken to hospital on both sides. A security source said at least 15 protesters had been detained.
Leadership calls for calm
President Michel Aoun ordered the army and security commanders to restore calm.
Saad al-Hariri, who resigned as premier in October, said the violence threatened civil peace. "It is an insane, suspicious and rejected scene," he tweeted.
No rescue plan in sight
After the unrest pushed Hariri to quit, feuding politicians have failed to agree on a new cabinet or rescue plan. The Lebanese pound has lost nearly half its value, dollar shortages have driven up prices and confidence in the banks has collapsed. The Internal Security Forces (ISF) said they were being "violently and directly" confronted on Saturday night. "Those who are rioting will be pursued, arrested and referred to the judiciary," it said on Twitter.
Young men chanting "revolution" hurled stones, steel barriers and flower pots at riot police. Protesters had tried to push into the heavily barricaded part of central Beirut which includes the parliament. Hundreds of people had marched and chanted against the political class in other parts of the capital in the afternoon. A large banner at one of the rallies read: "If the people go hungry, they will eat their rulers."