Greek riot police fired stun grenades and pepper spray to push back protesters throwing stones and petrol bombs in central Athens.
Tens of thousands of people took to the streets to protest a visit by German Chancellor Angela Merkel.
Blamed by many Greeks for imposing draconian budget cuts in exchange for aid, Ms Merkel reaffirmed Berlin's commitment to keep the debt-crippled state inside the eurozone.
She tried to reassure her hosts that their reforms would eventually pay off.
However, Ms Merkel also made it clear that Greece, which has seen its unemployment rate surge to nearly 25% and economic output shrink by a fifth, would not solve its problems overnight.
Greece is in talks with the Troika of lenders - the European Union, European Central Bank and International Monetary Fund - on the next tranche of a €130bn loan package, its second bailout since 2010.
Without the €31.5bn tranche, Greece says it will run out of money by the end of November.
Many Greeks say they cannot take more of the wage cuts and tax increases that have left a quarter of the workforce jobless and slashed the country's economic output by a fifth.
"I have come here today in full knowledge that the period Greece is living through right now is an extremely difficult one for the Greeks and many people are suffering," Ms Merkel said during a joint news conference with Mr Samaras just a few hundred yards from the mayhem on Syntagma Square, outside parliament.
"Precisely for that reason I want to say that much of the path is already behind us," she added, offering a public display of support to Mr Samaras's three-month-old government on her first visit to Greece since 2007.
Mr Samaras promised to implement economic reforms necessary to restore confidence: "The Greek people are bleeding but are determined to stay in the euro," he said.
On the other side of the parliament building, thousands of demonstrators defied a ban and gathered to voice their displeasure with the German leader, whom many blame for forcing painful cuts on Greece in exchange for the two EU-IMF bailout packages worth over €200 billion.
Greek police fired teargas and stun grenades when protesters tried to break through a barrier to reach the cordoned-off area where Ms Merkel and Mr Samaras were meeting.
Some demonstrators pelted police with rocks, bottles and sticks.
Four people dressed in World War Two-era German military uniforms and riding on a small jeep, waved black-white-and-red swastika flags and stuck their hands out in the Hitler salute.
Banners read "Merkel out, Greece is not your colony" and "This is not a European Union, it's slavery".
Some 6,000 police officers were deployed, including anti-terrorist units and rooftop snipers, to provide security during the six-hour visit.
German sites in the Greek capital, including the embassy and Goethe Institute, were under special protection.
Police fire teargas during French job protests
French riot police fired teargas to disperse protesters outside the Paris auto show as people marched nationwide to denounce hardship and job losses there.
Unemployment is at its highest in France since 1999 and economic growth has stopped.
The incident came as tens of thousands responded to a call by the CGT labour union, one of the two biggest in France, for the first nationwide protests since Socialist President Francois Hollande took office in May.
Police intervened after around 1,000 protesters, including workers from a doomed PSA Peugeot Citroen plant, attempted to break through a security cordon around the location of the car show on the edge of Paris, a Reuters reporter at the scene said.
Some protesters pelted police with eggs and flour during a standoff that lasted about two hours, ahead of a central Paris rally that drew upwards of 11,000 people, according to police.
The union put that number at 25,000.
France's unemployment rate stands above 10% and the number of jobless has topped 3 million for the first time in 13 years.