A car has been driven into a carnival parade in the western German town of Volkmarsen, injuring 30 people, including children.
Police said several of the injured were seriously hurt. Bild newspaper said a third of the injuries were serious and some were life-threatening.
The driver was in custody and being questioned, police said.
Regional broadcaster Hessenschau said police believed the driver, identified as a 29-year-old German citizen, had acted deliberately, but that nothing was immediately known about his motive.
German news website HNA cited witnesses as saying the driver appeared to have deliberately targeted children and had driven "at full throttle" into the crowd, which had gathered for the Shrove Monday procession.
The incident comes less than a week after a man gunned down 11 people, including himself, in one of the worst racist attacks in Germany since WWII.
All carnival parades in the western state of Hesse - in which Volkmarsen is located - had been called off as a precautionary measure, police said.
Carnival is hugely popular in parts of western Germany, especially in Rhineland cities such as Cologne and Duesseldorf, where festivities peak on "Rose Monday" with tens of thousands attending street parades featuring comical or satirical floats. Schools are often closed on the day, although it is not a national holiday in Germany.
Police cars and ambulances rushed to the scene in Volkmarsen, a small town in northern Hesse, 420km west of Berlin.
"We are on the ground with a big deployment. An investigation is underway," north Hesse police said on Twitter.
Amateur pictures published online showed police officers standing next to a silver Mercedes-Benz car that appeared to have been involved.
German media said the driver deliberately broke through plastic barriers set up by police around the parade area, where 1500 people were expected to gather.
The car had continued driving through the crowd for about 30 metres before coming to a halt in the incident, which took place at around 2.30 pm , an eyewitness told broadcaster Hessenschau.
In 2016, a Tunisian man with Islamist militant ties ploughed a truck into a Christmas market in Berlin, killing 12 people. He was later shot dead by Italian police after he fled Germany.
- Reuters / BBC