The country is suffering its second recession in three years, with a predicted 1.5 percent economic contraction in 2012. The Bank of Spain warned Wednesday the recession could be deeper.
Spain has come under pressure to tap the ECB bond-buying program that has been partly designed to keep a lid on the country's borrowing costs. So far, the government has been reluctant to ask for help for fear of the conditions that may be attached.
Spain's IBEX stock exchange fell in 4 percent Wednesday while the interest rate on its 10-year bonds rose 0.26 percentage points to 5.99 percent on concerns about the country's economy and that it is taking too long to make up its mind about applying for ECB assistance.
"The demonstrations remind us that central bankers cannot solve the crisis alone. The ECB's plan to intervene in sovereign bond markets can only succeed if governments in crisis countries can convince their electorates that ongoing austerity and reform are necessary to avoid bankruptcy," said Martin Koehring of the Economist Intelligence Unit.
"This, however, is increasingly challenging without the return of economic growth."
Greece, meanwhile, has been dependent since May 2010 on billions of euros in two rescue loan packages from other eurozone countries and the International Monetary Fund. In return, it slashed salaries and pensions and hiked taxes in an effort to reform an economy derailed by decades of overspending and corruption.
Although Greece accounts for only about 2 percent of the eurozone's total economy, its crisis has shaken the euro and led to concern it could destabilize other, much larger economies in the 17-nation bloc. Greece is in its fifth year of recession, with unemployment above 24 percent.
Shortly before Greece's strike got under way, Prime Minister Antonis Samaras and Finance Minister Yannis Stournaras hammered out a €11.5 billion ($14.9 billion) package of spending cuts for 2013-14 demanded by the country's international lenders, along with another €2 billion in improved tax collection.
The government has struggled to come up with austerity measures acceptable to the country's creditors, with disagreements arising between the three coalition parties. The next payment of €31 billion hinges on the further cuts.
Stournaras was briefing the other two party leaders Wednesday, and Samaras was to meet with them this morning. If they agree, the package will be presented to Greece's debt inspectors.
Wednesday's strike shut down Greece's famed Acropolis and halted flights for hours. Ferry services were suspended, schools, shops and gas stations were closed and hospitals functioned on emergency staff.
Government spokesman Simos Kedikoglou said the limited violence and what he called a smaller turnout than opposition parties had hoped for showed that "Greek society understands what the government is doing is the only possible solution."
Pan Pylas in London, Nicholas Paphitis and Derek Gatopoulos in Athens, and Alan Clendenning in Madrid contributed to this report.