The commotion started about 4:30 p.m. in the area of 34th Street and Lombard Avenue, said Everett police officer Aaron Snell.
A man and woman were involved in a domestic disturbance, Snell said.
The man reportedly threatened the woman with a knife and began chasing her down the street.
A passerby saw what was happening and feared for the woman's safety, Snell said.
The passerby, 61, of Arlington, had a pistol. He fired a shot in the air.
The man with a knife then stopped chasing the woman, Snell said. The man, 48, was detained by a group of people until police arrived. He was arrested for investigation of unlawful possession of a dangerous weapon.
Police also spoke with the man who fired the shot, Snell said. They don't expect to pursue criminal charges against him.
Officers on Wednesday night were trying to find the woman involved.
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