Athens, Greece (CNN) -- Protesters and police clashed violently in front of the Greek parliament building Wednesday, as tens of thousands of demonstrators gathered in Athens on the first day of a two-day general strike over austerity measures.
At least six protesters and 15 police officers were injured amid the disturbances, police said, and at least 15 people were arrested.
Strikers in Greece aim to shut down wide sectors of the country, as lawmakers debate a new round of tough cost-cutting measures.
Lawmakers passed the new austerity law in principal in an initial round of voting Wednesday, by 154 votes in favor to 141 against, with five lawmakers absent. Parliament must still vote on each article of the legislation Thursday before it can become law.
"Don't bow your head, it's time for resistance and struggle," marchers chanted in the capital earlier as they gathered for the union-backed demonstration.
The violence broke out around lunchtime in one corner of the square, beside Parliament House, as a group of protesters dressed mostly in black threw rocks and Molotov cocktails at police.
Officers fired tear gas and stun grenades, or "flash bangs," in return, sending noisy detonations echoing round the square.
Smoke filled the area by mid-afternoon as a fire burned in front of the finance ministry, forcing many peaceful demonstrators to move away.
Police estimated that more than 70,000 people were protesting in Athens, and said they planned to put between 2,500 and 3,000 officers on the streets. Organizers estimated the turnout at 120,000 people.
Initially, most of the protesters gathered peacefully in front of Parliament House waving union flags, red flags and banners.
"I'm here for my children and everyone else's children. Those punks in there have destroyed everyone's lives," said former railway worker Diamandis Goufas, 62, pointing at parliament.
Greeks are angry at yet another round of planned austerity measures as Greece tries to bring down its stratospheric debt.
Lawmakers are trying to cut government costs to reassure international backers it is doing enough to earn the bailout funds they have promised to pour into the country, with the latest austerity measures expected to pass Thursday.
The new bill would lead to around 30,000 job losses and further cuts to wages and pensions for workers in the public sector.
That has left at least some Greeks furious at the countries demanding that Greece bring down its spending.
"We are not lazy; it's the Germans, they want to take our blood," said Eleftherios Zarkados.
At least one student said Wednesday that Thursday would not mark the end of the battle between politicians and the public.
"We will continue to resist even if the measures pass," said Sophia Titou, 21, a law student who works at an oil refinery.
Many on the streets say they are angry that the well-off people they believe are benefiting from corruption and tax evasion are not being pursued, while public sector workers pay the price for Greece's woes.
European Union leaders are scrambling to minimize the effect of Greece's debt on their common currency, the euro.
Over the weekend, finance ministers from the world's largest economies pledged their commitment to take "all necessary actions" to stabilize markets.
They aim to keep banks well capitalized so they can weather the effects of any defaults by Greece or other indebted countries like Portugal, Spain, Ireland or Italy.
But there appears to be a split between France and Germany -- Europe's two largest economies -- on how to do it.
Germany has stressed that individual European states should inject capital into domestic banks that lack sufficient buffers. But analysts say France is opposed to this idea because it could jeopardize the nation's top-tier credit rating.
European leaders are expected to hear concrete details about how the plan might work at a European Council meeting Sunday.
European Union heads of state are widely expected to finalize the plan in early November at a meeting of the Group of 20 world economic powers.
CNN's Andrew Carey, Diana Magnay and Ben Rooney contributed to this report.