German chancellor Angela Merkel has said she is "shocked, shaken and deeply saddened" by the attack on a Berlin Christmas market which left at least 12 people dead.
Mrs Merkel told reporters it would be "particularly sickening" if it turns out the attacker was an asylum-seeker who sought refuge in Germany.
German media have reported that a suspect arrested after the attack was a Pakistani citizen who came to Germany in late 2015 or early 2016.
Police are treating the incident as a suspected terror attack after a driver rammed the truck into a crowded Christmas market in the heart of the German capital. Another 50 people were injured in the incident.
The vehicle struck outside Berlin's Kaiser Wilhelm Memorial Church late on Monday as tourists and local people were enjoying a traditional pre-Christmas evening near Berlin's Zoo station.
Berlin police said on Twitter: "Our investigators are working on the assumption that the truck was intentionally driven into the crowd at the Christmas market on Breitscheidplatz.
"All police measures concerning the suspected terror attack at Breitscheidplatz are being taken with great speed and the necessary care."
The suspect was picked up about a mile and a quarter away from the crash site.
Berlin's public radio station RBB-Inforadio cited security sources saying the man entered Germany on December 31 2015. News agency dpa, also citing unnamed security sources, reported that he arrived in Germany as a refugee in February 2016. Berlin's Tagesspiegel newspaper reported that the man was known to police for minor crimes.
Die Welt daily reported that police raided a large shelter for asylum-seekers at Berlin's defunct Tempelhof airport overnight.
Berlin police declined to confirm the reports, but spokesman Winfried Wenzel said the suspect was being interrogated.
Germany has not experienced any mass-casualty attacks by Islamic extremists, but has been increasingly wary since two attacks by asylum-seekers in the summer which were claimed by the Islamic State (IS) group.
Five people were wounded in an axe rampage on a train near Wuerzburg and 15 in a bombing outside a bar in Ansbach, both in the southern state of Bavaria. Both attackers were killed.
Those attacks, and two others unrelated to Islamic extremism in the same week-long period, contributed to tensions in Germany over the arrival last year of 890,000 migrants.
Far-right groups and a nationalist party seized on last night's attack, blaming Mrs Merkel for what had happened.
"Under the cloak of helping people Merkel has completely surrendered our domestic security," said Frauke Petry, co-chairwoman of the Alternative for Germany party.
Manfred Weber, a member of Mrs Merkel's conservative party and leader of the European Parliament's biggest political grouping, cautioned against reaching sweeping verdicts following the attack. But he said it was important to ensure that extremists did not enter the country among those seeking refuge in Europe.
"The state must be able to check every refugee who comes here," he told German public broadcaster ARD.
The White House condemned "what appears to have been a terrorist attack". It came less than a month after the US State Department called for caution in markets and other public places across Europe, saying extremist groups including IS and al Qaida were focusing "on the upcoming holiday season and associated events".
IS and al Qaida have both called on followers to use trucks in particular to attack crowds. On July 14, a truck ploughed into Bastille Day revellers in the southern French city of Nice, killing 86 people.
IS claimed responsibility for that attack, which was carried out by a Tunisian living in France.
After the Berlin attack, dozens of ambulances waited to evacuate people, and heavily armed police patrolled the streets. Authorities on Twitter urged people to stay away from the area, and to keep the streets clear for rescue vehicles.
Among the dead was a man in the truck, who succumbed as paramedics treated him, police spokesman Mr Wenzel said. Police said later that the man was a Polish national, but did not give further details of who he was or what happened to him.
Polish prime minister Beata Szydlo said it is "with pain and sadness we received the information that the first victim of this heinous act of violence was a Polish citizen".
She said the attack is a reminder that "Europe must become unified in the fight against terrorism and Europe must take effective action to protect its citizens".
The Polish owner of the truck said he feared the vehicle may have been hijacked. Ariel Zurawski said he last spoke with the driver, his cousin, T around noon, and the driver told him he was in Berlin and scheduled to unload ON Tuesday morning. "They must have done something to my driver," he told TVN24.
Federal prosecutors, who handle terrorism cases, took over the investigation, according to German justice minister Heiko Maas. In Washington, White House National Security Council spokesman Ned Price said the United States was ready to help in the investigation and response.
US president-elect Donald Trump said Islamic extremists must be "eradicated from the face of the earth" and pledged to carry out that mission with all "freedom-loving partners".