Dozens more were wounded Monday when the truck tore through the crowd, smashing through wooden stalls and crushing victims, in scenes reminiscent of July's deadly attack in the French Riviera city of Nice.
Police detained the man believed to have deliberately crashed the heavy vehicle loaded with steel beams into the festive market in a area popular with tourists near the capital's iconic Kaiser Wilhelm Memorial Church.
The man behind the wheel was an asylum seeker believed to be from Afghanistan or Pakistan who arrived in Germany in February.
A Polish man, thought to have been the truck's registered driver, was found dead on the passenger seat, and police said he had not steered the vehicle.
Twelve people were killed and 48 others injured as the lorry tore through the market for as far as 80 metres (yards) in the incident which came less than a week before Christmas.
"I don't want to use the word 'attack' yet, although there are many things pointing to one," said Interior Minister Thomas de Maiziere hours later.
Berlin police early Tuesday said they presumed the lorry was "intentionally steered" into the crowd and called the bloody carnage a "probable terrorist attack".
One of the survivors, Australian Trisha O'Neill, said she was only metres from "this huge black truck speeding through the markets crushing so many people".
"I could hear screaming and then we all froze. Then suddenly people started to move and lift all the wreckage off people, trying to help whoever was there," she told the Australian Broadcasting Corporation.
O'Neill said there was "blood and bodies everywhere".
'We mourn the dead'
Chancellor Angela Merkel reacted quickly, with spokesman Steffen Seibert tweeting: "We mourn the dead and hope that the many people injured can be helped."
A German police spokeswoman said that a man who was apparently driving the truck had been detained while the passenger was dead.
The daily Tagesspiegel said the man behind the wheel was known to police but for minor crimes, not links to terrorism.
Police later identified the passenger as a Polish national, while the Polish owner of the lorry confirmed his driver was missing.
Traditional Christmas markets are popular in cities and towns throughout Germany and have frequently been mentioned by security services as potentially vulnerable to attacks.
"It's awful. We were in Berlin for Christmas," said American tourist Kathy Forbes. "We also thought it would be safer than Paris."
The crash happened at a square at the end of the Kurfuerstendamm boulevard in the shadow of the Kaiser Wilhelm Memorial Church whose damage in a World War II bombing raid has been preserved as a reminder of the horrors of war for future generations.
Europe has been on high alert for most of 2016, with terror attacks striking Paris and Brussels, while Germany has been hit by several assaults claimed by the Islamic State group and carried out by asylum-seekers.
An axe rampage on a train in the southern state of Bavaria in July wounded five people, and a suicide bombing left 15 people injured in the same state six days later.
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.
The arrival of 890,000 refugees last year has polarised Germany, with critics calling the influx a serious security threat.
The attack in Berlin comes five months after Tunisian extremist Mohamed Lahouaiej Bouhlel ploughed a truck into a crowd on the Nice seafront, killing 86 people.
In response to the suspected attack in Berlin, France beefed up security at its own Christmas markets.
"The French share in the mourning of the Germans in the face of this tragedy that has hit all of Europe," President Francois Hollande said.
The Nice bloodshed -- as people were watching a fireworks display on the Bastille Day holiday on July 14 -- further traumatised a France already reeling from a series of jihadist attacks.
The United States labelled Monday's incident an apparent "terrorist attack" and pledged its support.
President-elect Donald Trump blamed "Islamist terrorists" for a "slaughter" of Christians in the German capital.