Anti-Obama protesters clash with Greek police in Athens
Greek riot police have used tear gas and stun grenades in Athens to disperse about 3,000 left-wing marchers protesting against a visit by US president Barack Obama.
The violence broke out as youths in motorcycle helmets and gas masks, armed with wooden clubs and petrol bombs, tried to break a police cordon in front of a barrier formed by police buses.
Rioters retired to the Athens Polytechnic university complex in the city centre, site of a 1973 student uprising, and engaged in running street fights with police. No injuries were reported.
Left-wing and anarchist groups who organised the protest had originally planned to reach the US embassy in another part of town.
But authorities banned demonstrations in a large area of the city centre, to ensure that protesters came nowhere near Mr Obama, who was attending a formal dinner at the residence of Greek president Procopis Pavlopoulos.
About 5,000 Communist party supporters took part in a separate, peaceful protest in central Athens, which did not challenge the police cordon, and about 1,000 people took part in a similar protest in Greece's second city, Thessaloniki.
Greek authorities have deployed more than 5,000 police for Mr Obama's visit.
The riot squad was on high alert for violence after an armed anarchist group called for "attacks and clashes" to disrupt the visit and leftist militants injured a police guard outside the French embassy with a hand grenade.
There is a strong anti-American tradition among Greek left-wingers, who still resent US support for Greece's 1967-74 military dictatorship.
Mr Obama's visit comes just two days before the country's main annual anti-American demonstrations, which commemorate the bloody suppression, by military authorities, of the Polytechnic pro-democracy uprising.
The small Popular Unity party, which took part in the main protest, described Mr Obama's visit as "a provocation, much more as it comes during the commemoration of the heroic Polytechnic revolt, where the US-driven dictatorship squashed the students with tanks".
Party leader Panayiotis Lafazanis also blamed the US for Greece's economic woes. The debt-crippled country depends on international bailout loans, and has been forced by its creditors to implement deep income cuts, tax hikes, welfare cuts and economic reforms.
"American imperialism has not changed," Mr Lafazanis said on Tuesday. "The US presidents and administrations have played - and still play - a leading part in the bailout-linked plundering of our country... and their interventions are drowning our part of the world in blood and creating refugee waves."
The last visit to Greece by a US president was by Bill Clinton in 1999 - also very close to the Polytechnic commemorations. It was marred by extensive street fighting between anarchists and riot police.