At least nine people died after a truck mounted the pavement and rammed into crowds at a Berlin Christmas market.
The driver attempted to flee but was apprehended by police, they added.
On social media, police also announced that the rest of the city was safe, but continued to urge people to stay indoors.
Another witness Mike Fox said the lorry missed him by only about three metres as it drove into the market.
The police tweeted they have arrested a suspect in the area of the truck and are trying to determine if it is the driver.
Sal Hernandez and Talal Ansari reported from New York City. In November, a man who had expressed interest in driving a truck into Times Square was arrested in Brooklyn. A truck drove through the crowded site of a Bastille Day celebration, killing 86 people and sending 303 people in total to the hospital.
Germany has been shaken this year by several assaults claimed by the Islamic State group and carried out by asylum-seekers.
German Chancellor Angela Merkel is being briefed on the incident by Interior Minister Thomas de Maiziere and the mayor of Berlin.