Riot police fired teargas and rubber bullets at Polish and Russian football fans who fought bloody running battles on the streets of Warsaw on Tuesday, hours before their teams met in a Euro 2012 match.
Groups of young men, some of them masked, pelted Polish officers with rocks, bottles and flares. Television footage showed one man lying motionless on the ground.
Authorities had been bracing for a confrontation at the game between the neighbouring countries, whose relations have been poisoned by centuries of conflict and the Soviet domination of Poland after World War Two.
Riot police were seen dragging people off as Polish fans chanted: "Russia whores, Russia whores" and "Hit the red trash with a hammer, with a sickle".
Some Poles displayed a banner saying: "Polish President murdered in Russia," referring to a plane crash in Russia two years ago that killed Poland's president and 95 others.
At least 100 people were arrested and 10 people, mostly Poles, needed medical treatment, though none of the injuries were life threatening, said Polish police spokesman Mariusz Sokolowski. Officers used water cannon, teargas and rubber bullets, he added.
Violence erupted as thousands of Russian fans, flanked by riot police, marched through the city and started crossing its Vistula river on the way to the stadium.
Groups of fans began to provoke each other with insults then small groups started to scuffle. Reuters witnesses saw one man hit on the head by an iron bar thrown through the air.
"You could see on both the Polish and the Russian sides that it was organised groups of hooligans, quite small groups," said Polish fan Maciej Kowalski.
A film on YouTube showed a Polish fan lying unconscious on the Poniatowskigo bridge surrounded by police and reporters.
Later, hooligans started throwing stones and forecrackers at the police near centre of Warsaw where more than 63,000 gathered the match on video screens. More people were arrested.
Poland's PAP state news agency said an advisor to Russian president Vladimir Putin was travelling to Warsaw after the clashes.
"We need to calm the situation," PAP quoted Mikhail Fedotov, Putin's advisor for civil society and human rights, telling Echo of Moscow radio. Fedotov could not immediately be reached by phone.
About 20,000 Russian fans were expected in Warsaw for the match.
So far at Euro 2012 there have been only isolated incidents of violence and only a handful of arrests.
But the Russian Football Association is already facing punishment by UEFA after its fans threw fireworks and displayed banners during Russia's opening match against the Czech Republic in Wroclaw last Friday.