Two separate rallies — one organized by the country’s two main labor unions and the other by the Communist Party — drew roughly 13,000 protesters, police officials estimated. The organizers said at least twice as many people gathered for the two demonstrations.

Men and women shouted “traitors” and “employees of Merkel,” a reference to Chancellor Angela Merkel of Germany, at riot police in central Athens, while a crowd of younger protesters chanted “cops, pigs, murderers” — the Greek anarchists’ slogan.

By early afternoon, sporadic clashes had broken out between riot police and dozens of masked youths, some wearing gas masks, who hurled chunks of stone at police officers guarding Parliament, at Athens University and outside luxury hotels on the fringes of the capital’s central Constitution Square.

Most international travel was halted, with all scheduled flights into and out of the country canceled, the national rail service was suspended and ferries remained in their ports. Public transportation in the capital and other major cities was to run on a limited service to enable workers to attend protest rallies. Tax offices, courts and schools shut down for the day and hospitals were operating with only emergency staff.

The strike was called by the country’s two main labor unions, which represent about 2.5 million workers and have led resistance to the latest measures. These include additional taxes, further cuts to civil servants’ pay and pensions and a controversial plan to cut 30,000 jobs in the public sector which employs about 10 percent of Greek workers.

Protesters sardonically invoked Mrs. Merkel’s name in reference to the central role played by Germany in resolving Greece’s fiscal crisis. Lawmakers in Germany, Europe’s largest economy, voted last week to expand the bailout fund for heavily indebted European countries, a necessary step in approving a second bailout for Greece, a 110-billion-euro package agreed to in principle by European Union leaders in July.

The Greek finance minister, Evangelos Venizelos, who is trying to convince foreign auditors that Greece is getting its financial house in order, said on Tuesday that the government could only meet a budget deficit target for 2011, revised up to 8.5 percent of gross domestic product from 7.6 percent, if the Greek public unites behind the cutbacks.

“If state mechanisms don’t work and if we don’t have the cooperation of the public, we may have problems with the 8.5 percent target,” Mr. Venizelos said. In a nod to widespread tax evasion, he also appealed to Greeks to pay their taxes.

But national solidarity has been in short supply. Protests against the new measures have been vehement and the two main labor unions already have called a second strike for Oct. 19, ahead of planned votes on the new measures in Parliament. The votes are expected to be close as the governing Socialist Party has a majority of only four in the country’s 300-seat Parliament and some lawmakers are said to be wavering.

The strongest opposition is from the civil servants whom the government depends on to push through many of the changes such as the collection of additional taxes. Angry public sector workers have staged sit-ins this week at several government offices, including the Finance Ministry and Labor Ministry, thwarting the efforts of foreign inspectors to complete their audit.

The results of the audit by officials of the European Commission, the European Central Bank and the International Monetary Fund, known as the troika, will determine whether Greece receives the latest in a series of rescue loans. Without the release of an 8 billion euro installment — part of the 110 billion euro rescue package — Greece will run out of money to pay state salaries and pensions by mid-November, Mr. Venizelos said on Tuesday. Government officials had said last month that state coffers would run dry by mid-October.

A decision on the disbursement of the funding, originally scheduled to be made on Oct. 13 by euro zone ministers, has been put off until November, Jean-Claude Juncker, the prime minister of Luxembourg and head of the euro zone finance ministers, said late on Monday, noting that the troika needed more time to complete its report.

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