At least 12 people were killed and 48 were wounded after a truck plowed into a Christmas market in Berlin on Monday evening, German police said.

Berlin police tweeted that "a suspicious person was arrested near #Breitscheidplatz," the site of the incident. Police said they are investigating whether the suspect was the truck driver.

A person who had been in the truck's passenger seat at the time of the incident was killed in the crash, police said. Authorities identified him as a Polish national. The nationality of the suspected driver is still unclear.

The video below appears to show the scene in the aftermath of the incident:

#BREAKING: one dead, multiple others injured after a Truck plows into the Christmas market in #Berlin pic.twitter.com/zj427xegaQ — Amichai Stein (@AmichaiStein1) December 19, 2016

The truck, which apparently belonged to a Polish transportation company and was loaded with metal rods, ran into the market outside the Kaiser Wilhelm Memorial Church about 8 p.m. local time on Monday.

The owner of the company told German newspaper Bild that he had not communicated with the driver of the truck since about 4 p.m. local time, approximately four hours before the incident. He said he assumed the truck was stolen.

Police and emergency workers stand next to a crashed truck at the site of an accident at a Christmas market on Breitscheidplatz square near Kurfuerstendamm Avenue in the west of Berlin, Germany, December 19, 2016. Fabrizio Bensch/Reuters

A bystander tweeted that "there is no road nearby," indicating she believed that the incident was not an accident.

"People were crushed," she said. "I am safe."

"We were so close and had to run away as people started shouting and running," Chloe Smith, a witness who was at the Christmas market at the time of the incident, told Business Insider via Facebook. "There were people running everywhere and loads of ambulances."

"If we had been there five minutes earlier, we would have been involved. So scary," she said. "Very shaken up, but we're safe at the hotel now."

Police stand near the Christmas market in Berlin, Germany, December 19, 2016. Pawel Kopczynski/Reuters

Police told German media that the first indications from the investigation into the crash suggest it was a "likely attack." In a tweet, German police urged people to stay indoors and refrain from spreading rumors about the incident.

"What we see here is dramatic," said Berlin's mayor, Michael Müller. "My thoughts are with the families of the dead and injured."

Security at Christmas markets in Germany and France was tightened following the incident.

Germany's interior minister, Thomas de Maiziere, said in a statement that his "thoughts are now with the relatives of the victims and of the injured of the terrible incident" and that he is in "direct and constant contact with the security officers for the state of Berlin and have offered any support from the federal police."

US President-elect Donald Trump condemned the attack in a statement Monday evening.

"Our hearts and prayers are with the loved ones of the victims of today's horrifying terror attack in Berlin," Trump said. "Innocent civilians were murdered in the streets as they prepared to celebrate the Christmas holiday. ISIS and other Islamist terrorists continually slaughter Christians in their communities and places of worship as part of their global jihad.

"These terrorists and their regional and worldwide networks must be eradicated from the face of the earth, a mission we will carry out with all freedom-loving partners."

Police stand guard near a Christmas market in Berlin, Germany, December 19, 2016 after a truck ploughed into the crowded Christmas market in the German capital. Pawel Kopczynski/Reuters

US National Security Council Spokesperson Ned Price said the US "condemns in the strongest terms what appears to have been a terrorist attack" on the Christmas market.

"We have been in touch with German officials, and we stand ready to provide assistance as they recover from and investigate this horrific incident," Price said in a statement. "Germany is one of our closest partners and strongest allies, and we stand together with Berlin in the fight against all those who target our way of life and threaten our societies."

For many, the episode was reminiscient of the July 14 attack in Nice, France, in which 86 people were killed and more than 400 injured when a truck plowed into a crowd watching Bastille Day fireworks.

The mayor of Nice, Christian Estrosi, tweeted about the attack on Monday: “Horror in Berlin. Support to the Berlin mayor. Never again.”

Barbara Tasch, Charles Clark, and Mark Abadi contributed reporting.