A suspect has been arrested after a truck plowed into a crowded Christmas market in Berlin on Monday. At least 12 people are dead and 48 injured.

Police said they are still investigating why the truck veered off the road into the market, tearing through tables and wooden stands.

Berlin's top security official, State Interior Minister Andreas Geisel, said he didn't want to speculate, but that the circumstances pointed to an attack.

The White House said it "appears to have been a terrorist attack."

Police have arrested the person suspected to be the driver of the truck that hit the crowd. Twelve people were killed and many others injured. (Fabrizio Bensch/Reuters)

White House National Security Council spokesman Ned Price said the U.S. was ready to assist Germany in investigating and responding.

The popular market is near the fashionable Kurfuerstendamm avenue in the west of Germany's capital city at the Kaiser Wilhelm Memorial Church. Dozens of ambulances lined the area waiting to remove the injured while heavily armed police patrolled the area.

A suspect believed to be the driver was picked up about two kilometres away, near the Victory Column monument. He was being interrogated, said Berlin police spokesman Winfried Wenzel.

Truck's plates are Polish

Wenzel said the truck was registered in Poland, but that police were still investigating where it came from and who the driver was.

The Polish owner of the truck said he feared the vehicle driven by his cousin may have been hijacked.

Ariel Zurawski said he last spoke with the driver around noon, and the driver told him he was in Berlin and scheduled to unload Tuesday morning. "They must have done something to my driver," he told TVN24.

Among the dead was a passenger in the truck, who succumbed as paramedics treated him, Wenzel said. Police said later that the man was a Polish national, but didn't give further details of who he was or what happened to him.

It was not immediately known if the man arrested or the passenger found dead in the truck was Zurawski's cousin.

A bystander captured the wreckage after a truck slammed into a crowd at a Christmas market in Berlin 0:33

Reaction has been trickling in from German officials including Justice Minister Heiko Maas, who said federal prosecutors were taking over the investigation. They are the team that handles terrorism cases.

German Chancellor Angela Merkel was briefed on the situation by the country's interior minister and the mayor of Berlin, according to a government spokesman.

Stay at home, police say

It has been less than a month since the U.S. State Department called for caution in markets and other public places, saying extremist groups including ISIS and al-Qaeda were focusing "on the upcoming holiday season and associated events."

A truck has crashed into a Christmas market in Berlin, killing nine and injuring many others. <a href="https://t.co/NsO7bR3Cdy">https://t.co/NsO7bR3Cdy</a> <a href="https://t.co/efYEkAbUNw">pic.twitter.com/efYEkAbUNw</a> —@CBCNews

Following the July attack in the French city of Nice, ISIS and al-Qaeda have called on followers to use trucks in particular to attack public places. Czech authorities said they were increasing security as a result of what happened in Germany on Monday.

On Twitter, Berlin police asked people to stay at home and not spread rumours or videos from the scene. Facebook has activated its safety check feature in the wake of the incident.

The Canadian government urged Canadians in Berlin to follow the direction of local authorities and call if they require emergency consular assistance.

'Horrifying' scene, witness says

A witness told CBC's As it Happens he was about 50 metres away when the truck smashed into the market.

"I was on the other side of the street in which the Christmas market took place," said Jan Hollitzer.

"I heard noise from destroyed small houses that were centred around the place, and some screams," he said, noting he did not initially see the truck enter the market. "Then the truck came out of the Christmas market again, destroyed some small house and came out on the street."

Hollitzer, a journalist, said he decided to document what was happening.

"I tried to put it in words, but those are pictures you don't want to see in your life," he said.

"The injured people on the ground and people under the truck. I don't want to go into too much detail because I've never seen this before and I hope I never have to see this again. It's really horrifying."

Hollitzer said he had a colleague who reported from the Nice truck attack, and drew parallels between what happened there and what happened in Berlin.

"That was what I first imagined when I saw that … 'Oh no. We have Nice, in Berlin.'"

A police officer speaks to an ambulance attendant near where the truck slammed into a Christmas market in Berlin. (Sean Gallup/Getty Images)

Speaking to CBC News from Berlin, freelance journalist Nick Spicer said the truck drove into the crowd around 8 p.m. local time, when the market was crowded with people.

"If you wanted to hurt a large number of people with a truck, this is the kind of place you would go to," Spicer said.

In a statement, U.S. president-elect Donald Trump, in reference to Berlin, said ISIS "and other Islamist terrorists continually slaughter Christians in their communities and places of worship as part of their global jihad."

He said they must be "eradicated from the face of the earth" and pledged to carry out that mission with the U.S.'s "freedom-loving partners."

On Twitter, he referred to the incident, as well as events in Switzerland and Turkey, as a terror attack.

Canada's Minister of Foreign Affairs Stéphane Dion on Twitter lamented the "tragic loss of life" and offered his condolences to the victims' families.

Today there were terror attacks in Turkey, Switzerland and Germany - and it is only getting worse. The civilized world must change thinking! —@realDonaldTrump

1/2: Shocked and saddened to learn of what appears to be a possible terror attack that led to tragic loss of life today in <a href="https://twitter.com/hashtag/Berlin?src=hash">#Berlin</a>. —@MinCanadaFA