The US Embassy in Berlin has issued a travel warning for Americans going to Hamburg.
German police last month cracked down on protestors which led to some of the worst rioting in Germany in years.
Over the weekend, continued skirmishes led the police to declare parts of Hamburg “danger zones”.
The unrest followed by an anarchist community centre being evicted by police. The Rote Flora centre had been established by squatters almost 25 years ago.
Protestors and police clashed several times during December which resulted in dozens of people, including police officers, being injured, some seriously.
Police now have special powers to stop and search people in the “danger zones”, including the red light area called Reeperbahn, the Old Town, St Pauli and Altona Nord where the Rote Flora centre is.
Leftist bloggers have hit back, accusing the police of effectively imposing “martial law” in these areas.
Hamburg is Germany’s second largest city and one of the wealthiest and most dynamic towns in Europe.
Americans have been advised to avoid demonstrations and public gatherings in the areas and to carry ID at all times because of the stop and search conditions in force.
"If stopped without proper identification, persons may be detained by police without further justification," the US embassy warns.
