German police on Saturday extended a major manhunt after a man stabbed three people to death and wounded eight others at a street festival in the city of Solingen.

Police closed off the center of Solingen, while victims and witnesses were being questioned about the attack on Friday night, according to a statement which said five of the injured were in "serious" condition.

Special forces were among security personnel in the city center while a helicopter flew overhead, according to an AFP journalist.