Escalating political turmoil in near-bankrupt Greece intensified concerns Wednesday that the Mediterranean nation may be spiraling toward a calamitous default with investors, potentially igniting a new phase in Europe’s debt crisis.

Global markets shuddered as embattled Greek Prime Minister George Papandreou launched a risky gambit to push his Parliament to pass another round of austerity measures. Failure to pass the cuts could lead the European Union and International Monetary Fund to withhold bailout money, leaving Greece short of cash to pay its creditors as early as next month — an event that some economists warn could destabilize the global financial system.

After thousands of protesters clashed with police as Parliament debated the measures in Athens, Papandreou tried and failed to forge a coalition government to ensure the package’s approval. In a late-night speech to the nation, he then said he would reshuffle his cabinet and call for a vote of confidence this weekend, wagering his job in an attempt to strong-arm politicians into passing the hugely unpopular package next week.

The spectacle spooked investors; in the United States, the Dow Jones industrial average fell 1.5 percent, almost 179 points, to 11,897.27. Markets around Europe dropped by a similar amount Wednesday, and continued the slide Thursday morning.

The political drama in Greece is playing out against deepening fears that the Mediterranean country poses a broader risk to the economic recovery underway in Europe and the United States. Some experts have compared a Greek default to the collapse of Lehman Brothers in September 2008, suggesting it could touch off a run of bank failures that could ripple across the globe.

The high stakes could yet force the Europeans to continue aiding Greece, and pressure mounted on E.U. leaders to end weeks of infighting over how and whether to continue propping Athens up.

The European Central Bank on Wednesday said that risks associated with high levels of government debt were now Europe’s “most pressing concern” and that the year-long effort to quell the crisis was “fraught with some detrimental shortcomings.” Though European officials have tried to contain the crisis by throwing bailouts to three small, troubled nations — Greece, Ireland and Portugal — investors are worried that far larger, heavily indebted nations such as Spain and Italy could be next. The sheer size of their economies could challenge the ability of the E.U. and IMF to bail them out.

Some rating agencies are now putting the risk of a Greek default as high as 50 percent. The effects could cascade through the global financial system, particularly in Europe, where major banks hold massive portfolios of government bonds from Greece as well as other financially troubled nations in the region. Emphasizing that point, Moody’s Investor Services on Tuesday said it may downgrade the credit rating of several major French banks because of their holdings of Greek government bonds and other investments in the Greek economy.

A Greek default could affect other governments as well, undermining confidence throughout the 17-nation euro zone and driving up the borrowing costs of countries across the region. U.S. banks generally have limited exposure to Greek debt but could be affected by problems in Europe’s banking system.

To forestall that, European and IMF officials have been in talks for weeks over a new aid package for Greece, which was provided with a $160 billion, three-year bailout last year that has proved inadequate. Growth has been less than expected, tax receipts have run behind schedule and government-promised changes have been slow to produce results.

Those talks, however, have produced as much discord as agreement. Along with forcing Greece to make further concessions — and triggering the political crisis — there has been a standoff between the European Central Bank and a group of European nations, most notably Germany, that say they will not lend Greece more money unless private investors contribute to the country’s rescue. The German finance minister has suggested that Greek bondholders extend the term of their investment by seven years, giving the country more time to invigorate its economy.

The political turmoil in Athens, however, has raised the prospect that the crisis in Greece could escalate fast.

“A political situation like this is the last thing you need when you’re already on the edge of a cliff,” said Ioannis Sokos, market analyst at BNP Paribas in London.

Though Papandreou — a U.S.-educated champion of efforts to rekindle economic growth — maintains a narrow majority in Parliament, some of his Socialist Party members have recently turned against his austerity drive. If he cannot bring them in line to support him in the confidence vote this weekend, his government may fall.

“He is going to win this,” said a Greek government official who was not authorized to comment publicly and spoke on the condition of anonymity. “Of course there is always a risk, but he is doing this for the country. He will pull through.”

Papandreou told the nation Wednesday that he was taking such drastic steps because the conservative opposition had rejected his offer to forge a new coalition government — an offer in which he said he would resign if his opponents agreed to vote for the austerity measures. The conservative opposition, however, has argued that the plan backed by the IMF and E.U. — particularly the raising of taxes — has succeeded only in driving the country further into recession, and instead it demanded a renegotiation of the bailout terms.

“I have learned in my life to fight for the country, for the economy, for the people,” Papandreou told the nation.

The IMF has said that disbursement of the next tranche of bailout funds — a little more than $3 billion from the IMF and nearly $9 billion from other European countries — is contingent on the Greek Parliament approving the new concessions.

Yet analysts said European officials, whose nations stand to lose the most in a Greek collapse, may ultimately prove more forgiving, perhaps finding a way to provide Greece with bridge funds if it is unable to pass the measures.

“European leaders want to stop this from spreading. Spain in particular is still very vulnerable,” Sokos said. “Even in a worst-case scenario, if the reforms don’t pass, that could push Europe to at least give enough money to keep Greece alive.”

Schneider reported from Washington.