Police fire tear gas and stun grenades at demonstrators in Athens who are protesting about Angela Merkel's trip to the country.
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With protests feared, the authorities had closed roads in central Athens and created an exclusion zone around the areas the German chancellor was due to pass through.
Demonstrators tried to break through a barrier to reach Mrs Merkel, but were stopped in their tracks by the police. Tens of thousands of people gathered in Syntagma Square, some pelting the police with rocks, bottles and sticks.
Others donned Nazi uniforms, riding around in a four-wheel drive vehicle and waving swastika flags, a reference to Germany's occupation of Greece during the second world war (see picture above).
Dozens of protesters were detained in what the authorities described as one of the biggest demonstrations the capital had seen in months.
Some 6,000 police officers are on duty, with anti-terrorist units and rooftop snipers providing security during the six-hour trip.
Officers in riot gear are lining the streets, a barricade is in place outside parliament and police are stationed outside the German embassy.
Protesters are allowed to demonstrate in front of parliament, but are banned from areas around the entrance to the building, along with the prime minister's office and the hotel where Mrs Merkel is staying.
It is Mrs Merkel's first visit to Greece since its economic problems became apparent in 2009 and the trip represents a gesture of support for fellow conservative Antonis Samaras' government as it seeks to impose futher cuts on a country that has been in recession for five years.
After talks with the Greek prime minister, she commended his government, but said more work was needed to reduce the country's debts and restore competitiveness.
"A lot has been accomplished," Mrs Merkel said. Mr Samaras said Greeks were "bleeding", but would stick to their reform pledges and were determined to stay in the euro.
Greece is holding talks with inspectors from the so-called troika (the EU, IMF and European Central Bank) in the hope of securing the next tranche of a 130bn euro loan package. Without the 31.5bn euros, the Greek government says it will run out of money by the end of November.
The additional cuts are likely to include further wage and pension reductions and tax increases. Many Greeks say they cannot take any more pain, with people's standard of living falling and unemployment at 24 per cent.
Eurozone finance ministers and the IMF discussed Greece on Monday, but failed to make significant progress in deciding how to move forward.
Jean-Claude Juncker, the chairman of the eurozone's 17 finance ministers, and Christine Lagarde, the IMF's managing director, said they were pleased with Athens' progress, but more still needed to be done.
Athens is seeking more time to meet its financial commitments, and while the IMF is sympathetic, some countries, including Holland and Finland, have concerns.