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."
Police spokesman Thomas Neundorf said the current number of fatalities included the lorry passenger and the background to the incident was not yet clear.
He added: "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."
Mr Neundorf confirmed that 50 people had been injured in the incident and nine killed. He said he could not confirm the nationalities of those involved.
The truck carried Polish number plates and investigators would work to determine if it was stolen or driven legally, he also said.
It is understood the incident took place at a market outside the Kaiser Wilhelm Memorial Church on Monday evening.
Images shared on social media showed a damaged black truck with a smashed windscreen and 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.
She told Sky News she only missed being caught in the chaos because she had climbed up some steps to take a seat.
Ms Rushton said: "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."
Speaking in the House of Commons, Brexit minister Robin Walker said the UK stands "shoulder to shoulder" with Germany and other European countries following the "appalling loss of life" in Berlin.
Shadow Brexit minister Paul Blomfield also expressed "solidarity" with the people of Germany.
The Foreign Office has warned Brits 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."
As the incident unfolded, Berlin Police urged people to stay at home and refrain from spreading rumours. European Union officials sent their condolences.
Julian King, European commissioner for the security union, tweeted: "My thoughts are with all those affected and their families in #Berlin tonight."
Meanwhile, president of the European Council, Donald Tusk said he was "deeply saddened" by news of fatalities in the German capital and said: "Europe stands ready to help".
A Foreign Office spokesman said: "We are urgently investigating reports of an incident near the Kaiser Wilhelm Memorial Church in Berlin and are in close contact with 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 also activated a safety check feature for travellers and locals on the social network.
Mr Neundorf said the lorry " drove into the Christmas market".
He added: "This lorry came from Kantstrasse. It went through the market stalls and then it left the area towards Budapesterstrasse.
"The driver of the lorry fled. We were able to arrest a suspicious person nearby.
"Now we are checking if this is the driver of the lorry."
In response to whether the incident was an accident or an attack, Mr Neundorf said it was unclear.
"The background to this incident is not yet clear," he said.
"The criminal police of the state of Berlin is currently investigating.
"All we can say at this current point in time is that there is no more danger.
"We are investigating how this lorry came to drive into the Christmas market, how did this situation happen?"
Dozens of emergency services were at the scene of the incident.
White House National Security Council spokesman Ned Price said the United States the incident appeared to be a terror attack.
In a statement, 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."
He said the US had been in contact with German officials, and was ready to provide assistance as the nation investigated and recovered from the "horrific incident".
Meanwhile, 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".