ATHENS — After weeks of relative calm, protests erupted Wednesday across Greece and demonstrators surrounded the Spanish Parliament for a second day to protest the austerity program of Prime Minister Mariano Rajoy.

Greek trade unions called a nationwide strike to contest billions of dollars in new cuts to salary and pension being discussed by the government and its international creditors. It was the first such walkout since a conservative coalition led by Prime Minister Antonis ­Samaras came to power in June.

Samaras is negotiating a $15 billion austerity package that is needed to persuade Greece’s so-called troika of lenders — the International Monetary Fund, the European Central Bank, and the European Commission — to release nearly $40.7 billion in financial aid that the country needs to stay solvent.

Earlier this month, the European Central Bank said it intends to buy unlimited quantities of debt from European nations. That had kept the peace in the financial markets until Wednesday, when political instability spooked investors, with the Spanish stock market dropping 3.9 percent and even the German DAX falling by 2 percent.

Rajoy has been trying for months to convince investors that Spain can handle its own problems and that it will not need a bailout, which would force Madrid to cede some authority over its fiscal affairs to its lenders. He said Spain is set to introduce more cutbacks to meet budgetary goals. Those will include restrictions on early retirement and various measures to streamline regulations and fight unemployment, he told The Wall Street Journal.

The proposed cuts in Greece have ignited new anger, with many talking openly of increased impoverishment as the nation grapples with a third round of austerity measures in three years. The demonstrations were peaceful throughout the morning, as civil servants, teachers, medical personnel, bank employees, and lawyers marched to the Athens center. A police spokeswoman put the turnout at 35,000 to 40,000 people.

But violence broke out shortly after 1 p.m., as a group of protesters wearing black face masks hurled gasoline bombs at police officers on Vasilissis Sofias, a wide avenue abutting the Parliament building. Firebombs were also thrown at the Finance Ministry and into the lush National Gardens.

Officers wielding batons responded with bursts of tear gas, scattering demonstrators and tourists as police helicopters circled overhead and flares exploded. Many cursed the police with cries of ‘‘traitors!’’ and ‘‘Merkel’s pigs!’’ — a reference to Chancellor Angela Merkel of Germany, which is widely blamed for insisting on strong austerity measures.

Samaras is scheduled to meet Thursday with his restive coalition partners — Evangelos Venizelos, the Socialist leader, and Fotis Kouvelis of the Democratic Left — to seek a common line regarding the $15 billion austerity plan.

Lenders’ representatives left Athens last week after tussling with the government over the depth of cuts for salaries and pensions. The two sides need to reach an agreement before the lenders can issue a report in October grading Greece’s ability to mend its tattered finances. Additional financial aid depends on a positive assessment.