By LIZ ALDERMAN

ATHENS — Tens of thousands of Greeks joined a second nationwide strike in three weeks on Thursday, moving to bring the country to a near-standstill in a bid to show European Union leaders meeting in Brussels that fresh austerity cuts being demanded by Greece’s lenders would cripple society and further depress an already battered economy.

Protest rallies began peacefully but were disrupted when demonstrators broke away from the crowd near Syntagma Square outside Parliament and threw rocks, bottles and firebombs at the police, who responded with tear gas. A crowd estimated by the police at 15,000 thinned out. Some people had tears streaming from their eyes. A rally by the Communist Party drew 7,000 more people, according to a police spokeswoman. Unions said the turnout was about 40,000 people, double the official estimate.

A 65-year-old demonstrator who fainted in central Athens around 2 p.m. died after unsuccessful efforts to give him cardiac resuscitation, according to a statement issued by the director of the capital’s Evangelismos Hospital. The doctor’s statement did not confirm whether the man’s heart problems were provoked by tear gas, as local press reports suggested.

Greek media outlets reported that five people suffered minor injuries, while more than 50 people were said to have been detained during the demonstration.

Many demonstrators shouted at the rows of the riot police officers. “You’re criminals, selling out your country for 600 euros a month.” One man screamed at a group of officers. “Why are you doing it? Why?”

It was the latest in a wave of protests that appear to be gaining steam in southern European countries weary of austerity, particularly Portugal and Spain, where citizens have come out en masse to push against grinding cuts as their economies spiral lower.

The action comes as Greece’s so-called troika of lenders — the International Monetary Fund, the European Central Bank and the European Commission — press Prime Minister Antonis Samaras to seal a package of austerity cuts of 13.5 billion euros, or almost $18 billion. Those include new cuts in salaries and pensions as well as demands to streamline rigid labor laws that are seen hurting the country’s competitiveness.

The package, which is needed to unlock a $41.3 billion loan installment that Greece needs to stay solvent, has been delayed several times in the last two months as the government butted heads with the troika, citing concerns that those at the margins of Greek society, and the rising numbers of people who are falling out of the middle class, cannot take much more. Leaders hope to agree to final details of the austerity plan in coming days.

“Agreeing to catastrophic measures means driving society to despair,” said Yannis Panagopoulos, the head of Greece’s largest private sector union. “The consequences as well as the protests will then be indefinite,” he said.

About 4,000 police officers fanned out in central Athens and near the Parliament building, where clashes broke out three weeks ago during the last nationwide strike.

The strike disrupted transportation throughout the country, with the subway in downtown Athens closed for much of the day and taxi drivers halting service for several hours. Flights were grounded for three hours in the morning when air traffic controllers staged a walkout, joining other civil servants and workers in the private sector, including lawyers, pharmacists and doctors.

Many shopkeepers joined the protest by closing their doors to protest a drastic fall in income, a sharp rise in taxes and a plunge in demand that have cost thousands of businesses and jobs.