One of the largest mass protests since the fall of Communism in 1989 hit Bucharest on Wednesday over deep government spending cuts committed to meet its IMF loan conditions.
Romanian authorities closed off parts of central Bucharest.
Police said 18,000-20,000 demonstrators had assembled by midmorning, shouting: "Down with the government", making the protest outside government headquarters one of Romania's biggest since mass protests forced Communist dictator Nicolae Ceausecu to flee.
Union leaders called for a general strike.
Romania's state sector, criticised for inefficiency and corruption, employs one-third of the working population and pensions, wages and other social benefits account for two-thirds of budget revenues.
Such spending has become increasingly hard to finance after triple-digit pay rises in the booming 2005-2009 period, as the private sector faces shrinking domestic demand and higher unemployment.
"We want the government to fall or they will destroy us," said Maria Vasile, a 42-year-old teacher carrying a whistle who was urging people to join the protest. "It's either us or them."
Romania has promised cuts to state wages of 25 percent and to pensions of 15 percent as part of an effort to meet International Monetary Fund requirements for the release of the next tranche of loans in a 20 billion-euro bailout package.