Hundreds of demonstrators also torched tyres, smashed windows and ransacked shops in an escalation of rioting during a summit of Nato leaders in the French city of Strasbourg.
The worst of the violence was centred close to the French side of the Bridge of Europe - a road link over the river Rhine which connects France with Germany.
The bridge is 5 km (3 miles) from the conference centre where 28 Nato leaders, including U.S. President Barack Obama, were meeting, and a pall of black smoke was clearly visible from the summit venue.
Reuters television crews said youths had set fire to an Ibis hotel, a one-storey border post and a third building that was not immediately identified.
A German first aide volunteer said around 50 people had been hurt in the clashes with riot police.
The demonstrators campaigning to have Nato disbanded following the end of the Cold War, had vowed to disrupt Saturday's meeting after two days of skirmishes with police.
However, organisers said they had wanted a peaceful protest and expressed shock at the violence.
"I am very angry. This was meant to be a peaceful demonstration ... and tonight, instead of images of peace we will see images of war," said Marie-George Buffet, a veteran leader of the French Communist party.
German police arrested 13 divers early in the morning shortly before leaders crossed the Rhine on a footbridge linking the two European neighbours, in a ceremony symbolising two generations of peace since World War Two.
Groups of protesters attempting to cross the Rhine from Germany via the nearby road bridge clashed briefly with French police and were forced back.
An Iranian journalist based in Germany collapsed with breathing difficulties earlier after being hit in with face with teargas and was taken away by ambulance, witnesses said.
Protest organisers said the majority of tens of thousands of activists in Strasbourg and nearby Baden-Baden did not aim to carry out violence and blamed tensions on security forces.
"No one here has attacked any police, but we have been hit with teargas and beaten up," said spokesman Monty Schaedel.
Police said some of the protesters who left a designated demonstrators' campsite near Strasbourg early on Saturday were armed with sticks, barbed wire and stones hidden in sacks.
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