Police said that several hundred protesters started to burn a makeshift barricade of slats of wood and trash bins on a road leading to a camp of demonstrators in the Strasbourg neighbourhood of Neuhof.
Demonstrators also threw firecrackers at police, causing burns to the legs of one and injuring another officer's hearing, police said. Both were taken to a hospital for treatment.
Some of the protesters said they were frustrated about the large police presence in the French city and across the Rhine river in the German cities of Kehl and Baden-Baden, where U.S. President Barack Obama and German Chancellor Angela Merkel met.
Protest organizer Moritz Volkatz asked people to return to the camp in Strasbourg, where some 3,000 people are staying, and to wait for Saturday's planned rallies.
Christoph Kleine, a spokesman for the protest network Block NATO, told The Associated Press earlier in the day that avoiding violence was one of the protesters' goals but they would eventually get from the camp to the heavily guarded centre of Strasbourg, where the NATO summit is being held Saturday.
"We do not want any clashes with police ... We will go around them, we will go through them," he said. "We will not attack them. But we will make our way."
Some 15,000 German police -- including 31 riot squads -- and 9,000 French police are on call for the summit.
German authorities had estimated that up to 25,000 protesters would take part in several demonstrations in Baden-Baden and Kehl. France's interior minister has suggested 30,000 to 40,000 ultimately could show up in Strasbourg, where the camp was set up to house demonstrators.
It was not immediately clear why the numbers, so far, had been so small.
In Baden-Baden, about 500 people -- mostly young adults -- walked quietly and slowly from the main train station to the city's centre, flanked on both sides by a massive phalanx of police, some in riot gear with thick pads, protective vests and helmets and batons. Police helicopters flew overhead.
"Sixty years is too long!" the protesters chanted. "No more NATO is the goal!"
There were about 5,000 police in the city where Obama arrived with his wife, Michelle, aboard Marine One, the presidential helicopter.
In Kehl, about 50 kilometres away, traffic was halted just after 12 p.m. across the Europe Bridge linking France and Germany.
A protest that had been scheduled there appeared to fizzle because so few people attended.
Strasbourg's streets were virtually deserted as unauthorized traffic was forbidden in many parts of the city. Schools and the university were closed. Most businesses and restaurants have shut down for the duration of the summit.
On Thursday night in Strasbourg, police detained at least 300 people and forced demonstrators back into a tent camp on the edge of the city.
Demonstrators destroyed telephone booths and attempted to build barricades before they were stopped, said a police spokesman.
On Friday, two of those were brought before a judge and charged with participating in an "armed gathering."
