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Petros Giannakouris, Associated Press
A woman walks past a line of riot police guarding the transport ministry during clashes with striking taxi drivers in Athens on Wednesday, July 20, 2011. Hundreds of taxi drivers took part in the protest against new licensing laws, as talks between the government and unionists collapsed. Taxi drivers have called an open-ended strike against the proposed changes, at the heart of Greece's vital tourism season. Greece is in the throes of an acute debt crisis, which European Union leaders will seek ways of tempering at a summit meeting in Brussels on Thursday.

ATHENS, Greece — Greek taxi drivers clashed with police Wednesday as they vowed to continue an open-ended strike that has caused big disruptions at airports and harbors during the peak of the country's vital tourist season.

Hundreds of taxi drivers protesting planned licensing reforms threw stones and water bottles at riot police guarding the transport ministry during negotiations with the government, and were dispersed with tear gas. The government is trying to make it cheaper for new cabbies to enter the business but talks over the plans collapsed Wednesday.

The three-day old taxi strike is the latest challenge to the Socialist government's program of reforms and painful cutbacks in response to a debt crisis that nearly saw the country slide into bankruptcy earlier this month.

Despite last year's €110 billion ($156 billion) bailout deal from its European partners and the International Monetary Fund, Greece is looking for another debt relief program. EU leaders are meeting Thursday in Brussels to discuss Greece.

"Tomorrow's summit will determine the future of the country and of Europe," government spokesman Elias Mossialos said.

Greek Prime Minister George Papandreou will meet European Commission president Jose Manuel Barroso in Brussels later Wednesday ahead of the summit. Over the past two days, Papandreou has spoken on the phone with Barroso, EU president Herman Van Rompuy and the prime ministers of Italy and Spain, as well as those of bailed-out Ireland and Portugal.

In a parliamentary briefing Tuesday, Finance Minister Evangelos Venizelos said the new program is expected to be worth an estimated €120 billion ($170 billion) — half of the money is expected to come from the eurozone and the IMF, a quarter from private sector involvement and the rest from Greece's ambitious privatization program.

Greece's 14-month old austerity program, that combines pension and public wage cuts with tax and retirement age increases, has sparked a wave of discontent around the country.

At times, it has turned violent, most notably earlier this month when lawmakers backed another austerity package required by international creditors for the country to get the next batch of its current bailout funds.

The taxi driver walkout has alarmed tourism industry officials, after strikers blocked access to the country's main airport and harbor for hours on Monday. Over the past two days, strikers have periodically shut access to regional ports, airports and highways, to protest the proposed licensing changes.

They claim the measures will lead to market saturation through a surge of new licenses and cut deep into their income.

"Our fight is just starting," union leader Efthymios Lyberopoulos said after the talks broke down.

On Wednesday, taxi drivers blocked access to Iraklio airport on the major resort island of Crete, and threatened to prevent cruise ship passengers from boarding tour buses to Cretan archaeological sites on Thursday.

In an apparent bid to deflect criticism the strike is harming tourism revenue — worth more than 15 percent of Gross Domestic Product — taxi drivers in the southern Peloponnese area took over ticket offices at the site and museum of Ancient Olympia, allowing visitors in for free.

The Socialist government, which is trailing the main opposition conservatives in polls, says the reforms are necessary under the terms of Greece's existing bailout deal.