What began as a peaceful protest ended in ugly scenes in the centre of Athens' tourist district as police blocked marchers from taking their demonstration against US foreign policy to the door of the American embassy.
The demonstrators showed their anger at the US-led war in Iraq
Angry scuffles between left-wing demonstrators and riot police were played out in front of hundreds of foreign tourists and journalists in town for the Olympics.
Marchers from the Stop the War coalition planned a peaceful protest against the visit to Greece of US Secretary of State Colin Powell, who they see as a figurehead for "ÜS imperialism".
But the sight of demonstrators' placards raining down on a police line and baton-wielding officers firing tear gas will be seen as a fresh blow to Greece's public image.
Before the march began, the Ministry of Public Order had warned that protestors would not be allowed to march to the embassy - on a route past the Hilton hotel where delegates from the International Olympic Committee are staying.
The coalition was adamant, however, that they would proceed as planned, although when the crowd gathered in the early evening sunshine, there were few signs that the night would turn violent.
As he handed out flyers, organiser Petros Constantinou told BBC News Online: "We are not anti-American, we are anti-imperialism. This is a peaceful protest at the provocative remarks he [Powell] has made on Iraq."
Riot police stood in rows three deep to block the protesters' path
Mr Constantinou, a writer for a union paper, had set up a bannered area with loudspeakers which were relaying activists' speeches to the crowd of a few hundred.
Demonstrators posed for dozens of photographers and camera crews - one man had trained his dog to hold a banner, which declaimed US foreign policy in Iraq.
Socialist Antni Tsekekou, 31, told BBC News Online she had come to "send a message to Colin Powell". She added: "We represent the opinion of a large majority of Greeks - most Greeks are against the war in Iraq."
Dozens of blue-shirted police officers flanked the crowd as it grew.
In the side roads, squads of riot police with helmets and shields waited out of sight while sergeants talked into radios.
When the speeches ended, the march began to the sound of Greek punk rock. Activists with loudhailers led chants of "Powell out" and other anti-American slogans.
Police were carefully marshalling the route of the march, but allowed demonstrators to head towards Syntagma Square, one of Athens' major tourist spots, on their planned route to the US embassy.
A protest that began peacefully ended with scenes of chaos
In the square itself, several hundred police and riot squad officers were waiting for the march, blocking access to some of the capital's most exclusive hotels.
Tourists and locals at the popular meeting place took photos and asked each other what was going on.
But it was when the march rounded the square onto the main road to the embassy - on one of the key Olympics transport routes - that the mood darkened.
Two dark blue police coaches, with wire grills over the windows, had been arranged as a roadblock. In front of them was a line of riot police three deep.
A stand-off ensued, with vociferous anti-American chants from the crowd.
As protestors pressed forward there was a brief melee - police and demonstrators clashed and for a few minutes police were pelted with bottles and placards from the crowd.
When the crowd pressed forward for a second time, police fired tear gas, forcing demonstrators back.
A young girl sat on a traffic island nursing a nosebleed. Further down the road, a pile of rubbish had been set on fire and bins knocked over.
As banner-waving activists retreated down the street from which they came, two elderly ladies at a bus stop looked at each other in bewilderment.
A father turned to his family and said in an American accent: "I guess they heard the US team won another gold."
Locking up his newspaper stall, a man who gave his name only as George, told BBC News Online: "This is not what all Greeks think - these people have got no respect for our country."