Nine people are confirmed dead and 50 injured after a truck ploughed into a crowded Christmas market in Berlin.
A "suspicious person" - thought to be the driver - had been arrested near the scene and a passenger in the lorry died "on the spot", authorities said.
Berlin Police tweeted: "A suspicious person was arrested near #Breitscheidplatz. Whether it is the driver of the truck, is currently under consideration.
"Currently, there are no indications of further dangerous situations in the city near #Breitscheidplatz."
As news of the incident spread globally, White House National Security Council spokesman Ned Price said the United States condemned it as an apparent terror attack.
He said: "The United States condemns in the strongest terms what appears to have been a terrorist attack on a Christmas market in Berlin, Germany, which has killed and wounded dozens."
President-elect Donald Trump branded it an act of terrorism, tweeting: "Today there were terror attacks in Turkey, Switzerland and Germany - and it is only getting worse."
But Berlin Police spokesman Thomas Neundorf said the background to the incident was not yet clear.
Mr Neundorf confirmed 50 people had been injured in the incident and nine killed, including the lorry passenger.
He said: "The nationality of the passenger and the driver is not known to me at the moment.
"This will all be the subject of the investigation."
The truck carried Polish number plates and investigators would work to determine if it was stolen or driven legally, he added.
The Polish owner of the truck, Ariel Zurawki, told TVN24 he feared the vehicle may have been hijacked and said "they must have done something to my driver".
The incident took place at a market outside the Kaiser Wilhelm Memorial Church on Monday evening, with images shared on social media showing a damaged black truck with a smashed windscreen among debris at the scene.
Briton Emma Rushton, who was in the market, saw the lorry rush past her at speed and said it could not have been an accident.
Ms Rushton told Sky News: "The stall that we bought our mulled wine from was completely crushed. People were tearing off wooden panels to get out."
She added: "It was not an accident. It was going 40mph, it was in the middle of the market. There was no way that it could have come off the road and it showed no signs of slowing down."
As the incident unfolded, Berlin Police urged people to stay at home and refrain from spreading rumours.
British Foreign Secretary Boris Johnson tweeted his condolences following the "terrible tragedy", while the Foreign Office warned Britons travelling to Germany of a high risk from terrorism.
In advice updated after the crash, it said: "There may be increased security in place over the Christmas and New Year period, including at Christmas markets and other major events that might attract large crowds.
"You should remain vigilant and follow the advice of local authorities."
Earlier, a Berliner Morgenpost reporter described the scene shortly after the truck ran into the crowd as eerily silent and said some injured shoppers sat in front of stalls and held each other.
It is understood the market is a regular festive treat for shoppers and includes stands that offer seasonal foods such as bratwurst, sweet waffles, candied fruits as well as mulled wine and homemade eggnog.
Facebook has activated a safety check feature for travellers and locals on the social network.
Dozens of emergency services were at the scene of the incident.