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Youths hurled rocks, flares and smashed-up paving stones at police outside the U.S. Embassy in Athens on Wednesday, during a mass rally to mark the anniversary of a 1973 anti-dictatorship uprising.

Riot police used tear gas and stun grenades during the brief but violent confrontation with dozens of anarchists, and chased groups that dispersed down streets near the embassy building. At least 23 people were arrested, authorities said, while three police officers were lightly injured and one protester was being treated in hospital for burns.

Groups of youths continued running clashes with riot police after the end of the march, while police helicopters with searchlights circled overhead.

More than 6,000 officers were on duty to monitor the annual demonstration, which was generally peaceful, with more than 20,000 people marching through central Athens. Another 12,000 protesters joined a rally in Greece's second-largest city, Thessaloniki, where anarchists attacked police with stones and petrol bombs after the end of the march. Riot squad officers responded with tear gas and stun grenades, and three suspected troublemakers were arrested.

Police also reported clashes in the southern city of Patras.

The march to the U.S. Embassy is to protest American support for the 1967-74 military rule in Greece, but this year coincided with harsh cost-cutting measures by the government, which is grappling with a serious debt crisis.

This year's demonstration took on an anti-austerity flavor, with protesters holding banners bearing slogans against the International Monetary Fund and European Union.

"Stand up to the IMF junta," read one banner, while demonstrators chanted: "We want to work, not the reforms and unemployment."

Greece's finances are under strict supervision by the IMF and the EU, who are extending the cash-strapped country a three-year euro110 billion rescue loan package that saved it from defaulting on its debts earlier this year. In return for the loans, the government has cut civil service salaries, trimmed pensions and increased taxes.

The government submits its full 2011 budget to Parliament on Thursday. The document is widely expected to include additional measures, although the prime minister has pledged these will not involve further salary cuts or new taxes.

In a message for the uprising anniversary, Papandreou argued cost-cutting reforms would bring long term benefits to Greek.

"We are changing the economy, making it viable, sustainable and competitive ... so that we will stand on our own feet and not be propped up by loans," he said.

The exact death toll from the events in November 1973, when the junta crushed the student uprising at the Athens Polytechnic, has never been definitively established, but some say dozens of people were killed in the days around the student uprising.

Ahead of Tuesday's demonstration, riot police were preventing access to the Polytechnic building — which is often used as a safe-haven by anarchist rioters seeking to evade the authorities. Under Greek law, police are not allowed into university campuses.

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Associated Press writers Derek Gatopoulos in Athens and Costas Kantouris in Thessaloniki contributed to this report.