A truck which ploughed into a crowded Christmas market in Berlin was deliberately crashed in a suspected terrorist attack, police said.
Twelve people died and 48 were injured - some seriously - when the vehicle rammed into a market taking place outside the Kaiser Wilhelm Memorial Church on Monday evening.
Berlin Police said on Twitter on Tuesday morning that the incident was intentional and a suspected act of terrorism.
"Our investigators are working on the assumption that the truck was intentionally driven into the crowd at the Christmas market on Breitscheidplatz," the post said.
"All police measures concerning the suspected terror attack at Breitscheidplatz are being taken with great speed and the necessary care."
A "suspicious person", thought to be the driver, was arrested near the scene, 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."
Berlin Police spokesman Thomas Neundorf said a passenger in the lorry - who officials later confirmed was a Polish national - was among those killed.
The truck carried Polish number plates and investigators would work to determine if it was stolen or driven legally, Mr Neundorf added.
The Polish owner of the truck, Ariel Zurawski, told TVN24 he feared the vehicle may have been hijacked and said "they must have done something to my driver".
Images on social media showed 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."
Jan Hollitzer, 36, deputy editor-in-chief of local news outlet Berliner Morgenpost, said the market fell "silent" following the truck attack as shocked visitors looked on the devastation.
He told the Press Association that he had seen "more than one" person lying under the vehicle.
As the incident unfolded, Berlin Police urged people to stay at home and refrain from spreading rumours.
White House National Security Council spokesman Ned Price earlier said the US condemned the incident 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 also branded it an act of terrorism, tweeting: "Today there were terror attacks in Turkey, Switzerland and Germany - and it is only getting worse."
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."
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.