ATHENS, Greece — After a period of relative calm, European markets shuddered once again Wednesday as protests erupted across Greece and demonstrators surrounded the Spanish Parliament for a second day to protest the austerity program of Prime Minister Mariano Rajoy.

Earlier this month, the European Central Bank announced its intention to buy unlimited quantities of debt from European nations, including the troubled economies of southern Europe. That kept the peace in the financial markets until Wednesday, when political instability startled investors, with the Spanish stock market dropping 3.9 percent and even the German DAX falling by 2 percent. The interest rate on the 10-year Spanish bond, which had been declining, inched closer to the ominous 6 percent level.

On Tuesday in Spain, tens of thousands of demonstrators besieged Parliament to protest austerity measures planned by Rajoy. Last week, more than half a million people marched in cities across Portugal to protest an increase in social security contributions, and a million marched in Barcelona calling for Catalan independence.

In Athens, trade unions called a nationwide strike Wednesday to contest billions of dollars in new salary and pension cuts 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.

Rajoy has been trying for months to convince investors that Spain can handle its own problems and that it will not need a bailout that would force Madrid to cede some authority over its fiscal affairs to its lenders, and is set to introduce new cutbacks to meet budgetary goals.

The proposed cuts in Greece have ignited new anger here, with many talking openly of increased impoverishment as the nation grapples with a third round of austerity measures in three years. The protests in Athens in the morning were peaceful, as civil servants, teachers, medical personnel, bank employees and lawyers made their way to the city center. A police spokeswoman put the turnout at 35,000 to 40,000 people — modest by Greek standards.

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 Parliament, sending bursts of flame and black smoke into the air. Firebombs were also thrown at the Finance Ministry and into the lush National Gardens next to Parliament.

Officers wielding batons responded with bursts of tear gas, scattering demonstrators and tourists as police helicopters circled overhead.