Ambulances and heavily armed police rushed to the area after the vehicle mounted the pavement of the market in a square popular with tourists, in horrific scenes reminiscent of July's deadly truck attack in the French Riviera city of Nice.
Berlin police have said that investigators assume it was a deliberate act, calling it "a probable terrorist attack". They have requested no photographs be taken as they move the truck.
"Our investigators assume that the truck was deliberately steered into the crowd at the Christmas market at Breitscheidplatz," police said.
Police said they had taken one suspect into custody while a Polish citizen found dead in the truck after it crashed into people gathered around wooden huts serving mulled wine and sausages at the foot of the Kaiser Wilhelm memorial church in the heart of Berlin.
Interior Minister Thomas de Maiziere refused to label Monday night's (Tuesday AEDT) deadly incident a terrorist attack, though he said "a lot points in that direction."
"We want to be very, very cautious and operate close to the investigation results, not with speculation," he told the ARD broadcaster.
The truck's owner, Ariel Zurawski, told Polish broadcaster TVN 24 that the driver, who was his cousin, had been transporting steel to Berlin and had not been reachable since 4pm.
He said he was sure his cousin was not an attacker. "It can't have been my driver," he said. "Something must have happened to him ... I am so shocked."
German police said later they were working on the assumption that the truck had been stolen from a construction site in Poland.
Police understand that the man found dead in the truck was not controlling the vehicle.
The nationality of the suspected driver, who fled the crash scene and was later arrested, was unclear, police said.
German media cited local security sources as saying that there was evidence suggesting the arrested suspect was from Afghanistan or Pakistan and entered Germany in February as a refugee.
Police later said that 48 injured people were taken to Berlin hospitals.
The daily Tagesspiegel said the man was known to police but for minor crimes, not links to terrorism.
German authorities said there was no indication of "further dangerous situations in the city near Breitscheidplatz", where the suspected attack took place.
German Chancellor Angela Merkel reacted quickly to the tragedy, with spokesman Steffen Seibert tweeting: "We mourn the dead and hope that the many people injured can be helped."
