The incident took place in Breitscheidplatz, a public square in western Berlin.
Jan Hollitzer, deputy editor in chief of Berliner Morgenpost, said: "I heard a big noise and then I moved on the Christmas market and saw much chaos.many injured people".
"It looked like [it was done] on goal", a police officer who witnessed the incident told Berliner Zeitung.
Berlin police issued caution and asked people not to "spread rumors" as they investigated the nature of the crash. The truck's windshield was smashed out on the sidewalk alongside the market, with a swarm of ambulances nearby.
The incident is reminiscent of an attack in Nice in July, when more than 80 died after a truck drove through late-night crowds celebrating Bastille Day on the seafront. Islamic State claimed responsibility for that attack, which was carried out by a Tunisian living in France.
On Twitter, the Zurich police department said: "Large operation after shooting at mosque in the Eisgasse". Officials said he had been planning the attack for months.
In another case, a 16-year-old German-Moroccan girl in February stabbed a police officer in the neck with a kitchen knife, wounding him badly, allegedly on IS orders.