A lorry ploughed into busy Christmas market in the heart of Berlin, killing at least twelve people and injuring up to fifty others, according to German police.
Ambulances and heavily armed officers rushed to the area after the driver mounted the pavement of the market in a square popular with tourists,
Police later arrested the suspected driver near the scene, and said that another passenger "riding in" the lorry had died. The lorry had a Polish license plate, police said.
Security sources cited by DPA news agency said that the man behind the wheel was an asylum seeker from Afghanistan or Pakistan who arrived in Germany in February.
The daily Tagesspiegel said the man was known to police but for minor crimes, not links to terrorism.
The Polish owner of the lorry confirmed his driver was missing.
"We haven't heard from him since this afternoon. We don't know what happened to him. He's my cousin, I've known him since I was a kid. I can vouch for him," transport company owner Ariel Zurawski told AFP.
Angela Merkel, the German chancellor, expressed her sympathy for the victims of the incident.
Germany has been on high alert for a major terror attack ever since Mrs Merkel opened the country’s borders to more than one million refugees.
Attack as it happened
German President Joachim Gauck said in a statement that it was an "awful evening for Berlin and for our country".
As emergency vehicles filled the area, Berlin police said they had "no indications of further dangerous situations in the city near #Breitscheidplatz".
Terror angle can't be discarded
The incident revived the grim memories of an attack in Nice in July this year where 86 people were killed when a truck was driven into a crowd by an extremist inspired by Islamic State of Iraq and the Levant (Isil).
According to the DPA news agency, police believe the lorry drove 50-80 metres (54-87 yards) through the market area during the incident, which occurred at 20:14 local time (19:14 GMT).
"The sequence of events points to either an accident or an attack," Mr Geisel said.
Heiko Maas, Germany's justice minister, said that federal prosecutors, who handle terrorism cases, are taking over the investigation.
He did not give further details in a post on Twitter on Monday night about the "shocking news" but added "we are mourning with the relatives" of the victims.
Both the Berliner Zeitung newspaper and the Berliner Morgenpost reported the truck ran into the market outside the landmark Kaiser Wilhelm Memorial Church this evening.
A photo posted by the Morgenpost showed damaged tables and stalls.
One social media user, Emma Rushton, said that people were "crushed", and that the lorry had been "going too fast" at around 60 kilometres per hour.