Chancellor Angela Merkel was being briefed by the Interior Minister and Berlin mayor on the situation, a government spokesman said.
Facebook activated its 'Safety Check' feature so its users in the city could identify as safe to family and friends.
However, police said there were no further indications of "dangerous situations" elsewhere in the city.
The authorties urged people to stay away from the area.
Berlin Christmas market lorry attack
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Mike Fox, a tourist from Birmingham, England, told The Associated Press at the scene in Berlin that the large truck missed him by about three meters as it drove into the market, tearing through tables and wooden stands.
"It was definitely deliberate," Mr Fox said, adding that he helped people who appeared to have broken limbs, and that others were trapped under Christmas stands.
Dozens of ambulances lined the streets waiting to evacuate people, and heavily armed police patrolled the area.
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