Thousands of people were gathering in central Stuttgart to protest against preventive measures against the coronavirus on Saturday, where police said the majority failed to follow health regulations.
"We advise participants to comply," Stuttgart police said in a tweet.
They said they had deployed helicopters to monitor the events, adding that violations of mask-wearing and social distancing rules would also be documented.
Meanwhile there was a festive atmosphere on the Marienplatz square, according to a dpa photographer.
Several hundred officers were policing the demonstrations. A police spokesman said he expected there would be many violations of the regulations.
City representatives said they would call off the events if people failed to follow the health guidelines.
Alongside several smaller rallies, thousands of people gathered ahead of a rally planned for 4 pm (1400 GMT) of the so-called Querdenken (Lateral Thinking) movement, which is made up of Covid-19 deniers, right-wing activists and anti-vaccination campaigners.
Some 2,500 people were expected to attend an event at the Cannstatter Wasen festival area on the banks of the Neckar river.
The Querdenken rally organizers said there were 6,000 people there.
The Querdenken movement is currently subject to monitoring by the domestic intelligence serivce, the Office for the Protection of the Constitution, for the state of Baden Wuerrtemberg, of which Stuttgart is the capital.
Meanwhile counter-demonstrators sought to block the rally. Police said their faces were covered and they sat in the road or on bikes, blocking the route. Police said they would carry them away if necessary in order to free up the road.
