German Chancellor Angela Merkel has said "we must assume" the deaths of at least 12 people at a Christmas market in Berlin "was a terrorist attack", saying it would be "repugnant" if it was true that a refugee was responsible.
A truck drove into the crowd at the market, injuring some 48 other people. A man has been detained in relation to the incident, which police believe may have been a deliberate act.
However the suspect, identified as a 23-year-old Pakistani refugee called Naved B, has reportedly denied involvement in the incident.
According to a live translation of her remarks at a press conference, Ms Merkel said: "We don't know anything for certain, but we must assume it was a terrorist attack.
"It would be very difficult for us to learn that a human being who came to Germany to ask for refuge and asylum committed this deed.
"It would be terrible for all Germans who are very active, day by day, in helping asylum seekers and refugees.
"It would be repugnant for those who are helping people who came to this country asking for our help."
Ms Merkel said she was "shocked and very saddened" by what had happened.
"Twelve people who were among us yesterday, who were looking forward to Christmas and who had plans for the festive season, are no longer among us," she said.
"It is a terrible deed which one cannot understand. It took their lives. Many people are injured, are fighting for their lives and their health.
"In these hours, first and foremost I think of these people, the dead, the injured, their families and friends. The whole of the country is with you in deep sadness."
Saying she would visit the site of the alleged attack and also hold a meeting of her security cabinet, Ms Merkel urged people not to allow terrorism to change their way of life.
"Millions of people, including myself, are asking ourselves, how can you live with the fact that, while celebrating the festive season where we want to celebrate life, somebody has come along and taken so many lives. I only know that we do not want to, and we cannot live with it," she said.
"We do not want to allow ourselves to be paralysed by terror. It might be difficult in these hours, but we will find a strength to continue living life as we want to live it in Germany, in freedom and openness and together."
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