The US embassy in Berlin has issued a travel warning for Americans visiting Hamburg after a police crackdown on leftist protesters in the German port.
Americans are advised in the warning to avoid demonstrations or public gatherings in the areas affected, which include the main nightlife district.
They are also urged to carry ID because of police stop and search powers.
After some of the worst rioting seen in Germany in years, police declared parts of Hamburg "danger zones" on Saturday.
The unrest has centred on the eviction of an anarchist community centre, the Rote Flora, which was set up by squatters in a former theatre nearly 25 years ago.
Thousands of demonstrators turned out for a protest on 21 December at which dozens of people, including police officers, were injured in clashes.
On 28 December, masked rioters pelted a police station with bottles and stones. Three policemen were injured, at least one of them seriously.
'Not that bad'
Police now have special powers to stop and search people in the gefahrengebieten, or "danger zones", which are in force 24 hours a day and cover the red light district of the Reeperbahn, the Old Town, St Pauli and Altona Nord, where Rote Flora is located.
"If stopped without proper identification, persons may be detained by police without further justification," the US embassy warns.
Since the "danger zones" were declared, leftist bloggers have accused the police of effectively imposing "martial law".
The travel warning for Hamburg, Germany's second biggest city and one of the wealthiest and most dynamic towns in Europe, was greeted with some incredulity by Germans.
"The US called a travel warning for my city of Hamburg!?" wrote one blogger on Twitter. "Honestly? That sounds like we were living in a civil war area. It's not that bad."
"Travel warnings from the US embassy for South Sudan, Libya and the Hamburg Danger Area," another tweeter quipped.
Current UK government travel advice for Germany does not mention Hamburg specifically but notes that travellers should carry their passports with them at all times as German police have the right to ask to see identification.