A truck has ploughed into a crowd close to a Christmas market in the German capital Berlin, killing 12 people and injuring up to 48 others, police say.

Key points: Truck crashed into a crowded Christmas market in Berlin's Breitscheidplatz square

Truck crashed into a crowded Christmas market in Berlin's Breitscheidplatz square Police say at least 12 people have been killed and 48 others injured

Police say at least 12 people have been killed and 48 others injured A suspect has been arrested but it is not known if he was the driver

A suspect has been arrested but it is not known if he was the driver A second person inside the truck died during the attack, police say

Police said first indications pointed to an attack on the market, situated at Breitscheidplatz square at the foot of the Kaiser Wilhelm memorial church that was kept as a ruin after World War II.

The truck, which had Polish license plates and was carrying steel beams, careered into the market at what would have been one of the most crowded times, when adults and children would be gathering in the traditional cluster of stalls.

Police said on Twitter that they had taken one suspect into custody and that a passenger from the truck had died as it crashed into people gathered around the wooden huts serving mulled wine and sausages.

The nationality of the suspected driver, who fled the crash scene and was later arrested, was unclear, police said.

Police said the passenger who was found dead in the truck was a Polish national. Police later said on Twitter the passenger was believed to have had no control over the vehicle, which they assume was "intentionally controlled into the crowd".

Police traced the truck used to a construction site in Poland, with the owner fearing the vehicle may have been hijacked.

An injured person is loaded into an ambulance at a Christmas market on Breitscheidplatz square in Berlin. ( Reuters: Fabrizio Bensch )

Witnesses describe terrifying scene

Australian woman Trisha O'Neill witnessed the carnage first-hand.

"I could hear screaming and then we all froze. Then suddenly people started to move and lift all the wreckage off people, trying to help whoever was there.

"Eventually we had to and there was blood and bodies everywhere.

"People, including children and older people, weren't moving — people were crying and I just burst into tears."

Witness Mike Fox told The Associated Press the truck missed him by only about three metres as it drove into the market, tearing through tables and wooden stands.

"It was definitely deliberate," said the tourist from Birmingham, England.

Sorry, this video has expired Tourist witnesses truck ramming crowd

He said he helped people who appeared to have broken limbs, and that others were trapped under Christmas stands.

US had warned about markets The US had warned its citizens about going to Christmas markets in Germany. They're very open. They're everywhere in the big cities, the small villages. They're a great place for people to carry and gather to buy gingerbread, decorations for trees, presents for loved ones. They are places out in the open where a lot of people would attend, and the US Department of State knowing that there was a high state of alert and these could be potential targets, had warned citizens that visiting these could not be a good idea. They're saying they had credible information that ISIS or al-Qaeda were planning attacks and focusing on the upcoming holiday season. - Europe correspondent Steve Cannane

"I heard a big noise and then I moved on the Christmas market and saw much chaos ... many injured people," Jan Hollitzer, deputy editor in chief of Berliner Morgenpost, told CNN.

"It was really traumatic."

Police cars and ambulances converged quickly on the scene as a huge security operation unfolded.

Emma Rushton, a tourist visiting Berlin, told CNN the truck seemed to be traveling at about 65km/h.

Asked how many were injured, she said that as she walked back to her hotel, she saw at least 10.

Firefighters stand beside the truck and debris of the Christmas market. ( Reuters: Pawel Kopczynski )

Scene reminiscent of Bastille Day attack in Nice

The incident evoked memories of an attack in Nice, France in July when a Tunisian-born man used a truck to mow down people watching fireworks on Bastille Day, killing 86.

That attack was claimed by Islamic State group.

Interior Minister Thomas de Maiziere said the circumstances of the crash were still unclear, adding: "I don't want to use the word 'attack' yet although a lot points to that".

US President-elect Donald Trump condemned what he called an attack, linking it to "Islamist terrorists" before German police officials had said who was responsible.

Australian Prime Minister Malcolm Turnbull also condemned the "attack", saying he had been in touch with security agencies and established all staff at the embassy in Berlin were safe.

"We are also intensifying the steps that we have already set in place to ensure that we have the protective measures to respond to attacks such as that that we've seen in Berlin," Mr Turnbull said.

Germany has not recently suffered a large-scale attack from Islamist militants like those seen in neighbouring Belgium and France.

But it was shaken by two smaller attacks in Bavaria over the summer, one on a train near Wuerzburg and another at a music festival in Ansbach that wounded 20 people. Both were claimed by Islamic State.

Julian Reichelt, editor in chief of Bild Berlin, said that there was currently a massive security operation under way.

"The scene certainly looks like a reminder of what we have seen in Nice," Reichelt said.

People urged to stay away from square

Sorry, this video has expired The truck crashed into a Christmas market outside the Kaiser Wilhelm memorial church in west Berlin. (Photo: Reuters/Pawel Kopczynski)

Berlin police have urged people to stay away from the area and use Facebook's safety check feature to let loved ones know they are safe.

The Department of Foreign Affairs and Trade (DFAT) said it was checking to see if any Australians had been injured.

"The Australian Embassy in Berlin is making urgent enquiries to determine whether any Australians have been affected by the incident at the Berlin Christmas markets," DFAT said in a statement.

A government spokesman said Chancellor Angela Merkel was being briefed by her interior minister and the Berlin mayor on the situation.

"I'm deeply shaken about the horrible news of what occurred at the memorial church in Berlin," Foreign Minister Frank-Walter Steinmeier said.

"Many people who visited the Christmas market today have died and even more are injured."

A patient is attended to in an ambulance after being injured in the truck attack. ( AP: Michael Sohn )

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