Two men have died after being injured by fireworks during New Year celebrations in Germany.

Police said a 35-year-old was killed after igniting fireworks and a 19-year-old suffered fatal head injuries as he set off a homemade device in separate incidents in the Brandenburg region outside Berlin.

Detectives have opened criminal investigations into "improper use" of pyrotechnics.

Multiple other fireworks injuries were reported across the country. An 11-year-old boy was flown to hospital by helicopter with serious facial injuries after being struck in the face by a firework thrown by a group of teenagers also in Brandenburg, while a 14-year-old girl suffered serious eye injuries when fireworks were lobbed at a group of people in the Thuringia region.

Hand surgeons at Berlin's trauma hospital worked continuously in three operating rooms through the night, treating 21 people, including five who required amputations.

However, the was no repeat of the mass gropings that marred New Year's Eve in Cologne two years ago amid heightened security to protect women from sexual harassment.

The city's police said it had received seven complaints of sexual harassment, while Berlin police reported 13 and seven arrests as several hundred thousand people celebrated at the capital's Brandenburg Gate.

There were also terrorism fears in the wake of the attack on 19 December, 2016, when an Isis militant drove a truck into a Berlin Christmas market, killing 12 people.

Police barred large bags from barricaded-off pedestrian party areas in Berlin and Frankfurt.

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Six officers in Berlin suffered temporary hearing loss when a firework was thrown at them during the arrest of a suspect. The 22-year-old man was believed to have thrown a firecracker that blew a hole in a police car's rear windshield.

Police said they also arrested a 16-year-old girl after she repeatedly threw fireworks at police and confiscated 44 illegal pyrotechnic devices they found in her possession.