"I heard a big noise and then I moved on the Christmas market and saw much chaos...many injured people," Jan Hollitzer, deputy editor in chief of Berliner Morgenpost, told CNN.
German media, citing police at the scene, said first indications pointed to an attack on the market, situated at the foot of the ruined Kaiser Wilhelm memorial church, which was kept as a bombed-out ruin after World War II.
Another reporter for the Berliner Morgenpost described it as a "gruesome scene".
Police cars and ambulances converged quickly on the scene as a huge security operation unfolded. The fate of the driver of the truck was not immediately clear, but Bild newspaper said he was on the run.
Emma Rushton, a tourist visiting Berlin, told CNN the truck seemed to be travelling at about 65km/h.
Asked how many were injured, she said that as she walked back to her hotel, she saw at least 10.
Julian Reichelt, editor in chief of Bild Berlin, said that there was currently a massive security operation underway.
"The scene certainly looks like a reminder of what we have seen in Nice," Reichelt said.
The incident evoked memories of the Nice attack in July when Tunisian-born man drove a 19-tonne truck along the beach front, mowing down people who had gathered to watch the fireworks on Bastille Day, killing 86 people. The attack was claimed by Islamic State.
-AAP
