Stock markets sank Thursday in Greece, Portugal and Spain amid worries over the European Union's debt woes, with Greek customs and tax officials walking off the job in opposition to cutbacks aimed at digging the government out of a budget crisis that has shaken the entire EU.

The 48-hour Greek customs strike is expected to choke imports until next week, with fuel supplies the most likely to suffer. Lines of trucks were already forming at the country's borders, with customs workers allowing through only those carrying perishable goods or pharmaceuticals.

The government of Socialist Prime Minister George Papandreou is under intense pressure from markets and other EU governments to bring its budget deficit down from 12.7 percent of economic output last year to 2 percent in 2013.

A Greek default would be a serious blow to the shared euro currency, but Greece and the EU have insisted that will not happen. Doubts about Greece's finances have also affected market sentiment toward the debt of Portugal and Spain, two other eurozone countries struggling with deficits.

Markets appear worried that political resistance to cutbacks will keep Greece and Portugal from sticking to their plans.

In Portugal, the minority Socialist government is facing an uphill fight against opposition parties which want to hike some areas of spending. All opposition parties are pushing for an increase in the amount provided to poorer regions of the country, and together they can out-vote the government in Friday's parliamentary session.

The minister for parliamentary affairs, Jorge Lacao, warned of "serious political consequences" if Parliament passes the opposition proposal.

"This obviously raises a problem of governability at a time when it is absolutely indispensable for the state to show it is committed to imposing discipline on public finances," Lacao said.

Share prices on the Lisbon Stock Exchange sank by more than 4 percent, driven down by concerns about the national debt and political resistance to that government's efforts to get its large budget deficit under control.

The benchmark PSI-20 index dropped 4.5 percent midmorning before recovering to a fall of 3.2 percent.

Spain's finance minister dismissed worries that her country's debt and other economic problems pose a Greek-style risk for the eurozone. Elena Salgado criticized EU Economy Commissioner Joaquin Almunia for having said Spain and Portugal have problems similar to those of Greece. But markets remained skeptical with the Ibex-35 index at the Madrid stock market down 2.6 percent at midday.

Salgado said "there is no comparison" between the Spanish and Greek situations.

Spain said last week that its 2009 deficit stood at 11.4 percent of GDP _ nearly four times the EU limit. The government announced a four-year austerity program with euro50 billion in spending cuts to try to bring the deficit back down to 3 percent of GDP in 2013.

The cost of insuring against losses on Portuguese government debt surged to an all-time high, underlining market concerns that the country will have trouble financing its ballooning deficit. The drop follows news that a government bond issue had to be reduced Wednesday in light of the rising cost of borrowing.

This week, the EU backed the Greek Socialist government's austerity plan, which includes a civil service pay freeze, higher retirement ages and cuts in special bonuses that make up a large part of civil servants' income.

EU Economy Commissioner Joaqin Almunia called the program "achievable" and a European Commission report pressed Greece to tackle problems of competitiveness, including high public and private sector wage levels.

The Athens Stock Exchange was down 2.29 percent in afternoon trading Thursday, while spreads on Greek government bonds over the equivalent German bond hovered around the 350 point mark.

"The key question that remains unanswered today, is quite where the money would come from to support the likes of Greece and Portugal if the capital markets prove reluctant to provide it," said Simon Derrick, a senior currency strategist at Bank of New York Mellon.

With Germany apparently unwilling to lead a bailout, "the outcome of this apparent stalemate is that, amazingly, the unpalatable topic of an IMF bailout seems to have re-entered the discussion," Derrick wrote.

The International Monetary Fund's chief said the multinational lending agency was ready to help Greece solve its debt crisis if asked to, but that he did not think Greece was headed for bankruptcy.

"We are there to help," Dominique Strauss-Kahn told the French radio station RTL. "I have a mission on the ground to provide technical advice requested by the Greek government. And if we're asked to intervene, we will."

But he added, "I understand that the Europeans don't want this for the moment."

EU and Greek officials have said Greece will not need a bailout _ although this has not stopped speculation that other EU countries might be forced to step in with assistance. EU leaders last May doubled to euro50 billion a crisis fund to bail out the 11 EU members that do not use the euro; the Lisbon Treaty which came into effect Jan. 1 allows EU nations to use EU money to bail out a troubled member but does not say how.

Strauss-Kahn expressed confidence that Papandreou's government would take the "necessary but extremely difficult" steps to solve the crisis.

On Tuesday, Papandreou broadened his austerity plan, announcing a blanket civil servant pay freeze, an increase in fuel taxes and a hike in retirement ages _ although he has not given details of the new measures.

But unions, which had so far been muted, have begun reacting. The customs and tax officials' strike will be followed next week by a nationwide 24-hour civil servants' strike on Wednesday, while one of Greece's largest umbrella unions, GSEE, will call a 24-hour strike on Feb. 24.

Farmers have already been blocking major highways across the country on and off for more about three weeks, frequently closing the northern border with Bulgaria and hampering the transportation of goods. They are demanding financial help to overcome low food prices, but the government has repeatedly insisted there is no money to spare.

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Associated Press writers Barry Hatton in Lisbon, Greg Keller in Paris, Daniel Woolls in Madrid and Aoife White in Brussels contributed to this report.