The man reportedly tried to flee after the crash, but he couldn't get away because uninjured people in the crowd grabbed him and held him. Police arrived on scene quickly and took the man into custody.
Seattle Police Detective Renee Witt said the man, believed to be in his early to mid 20s, was later booked into King County jail for investigation of vehicular assault.
Alcohol may have been a contributing factor in the collision, she said.
Police officials said the crash happened at about 2 a.m. at 36th Street and Phinney Avenue North as the man drove a Subaru Forester into a crowd of about 30 people, hitting six pedestrians.
One witness, who asked not to be identified, said the car was traveling "fast - very, very fast" as it approached the crowd.
"I see the people - everybody fly like bowling pins," he said.
"It was shocking, it was horrifying," said Steven Standard, another witness who was walking in the area and saw the immediate aftermath of the crash. "I saw four people laying in the road, and saw one person just walking around, bloody."
Five men and one woman were injured. The most seriously injured was a 21-year-old man who was rushed to Harborview Medical Center in Seattle with potentially life-threatening injures. He is listed in critical condition.
The other victims:
A 22-year-old woman was listed in serious condition at Harborview with multiple fractures.
A 31-year-old man was treated was listed in stable condition at Harborview with a fracture.
A 37-year-old man was listed in stable condition at Harborview with a fracture.
A 21-year-old man was listed in stable condition at Harborview.
A 28-year-old man sustained minor injuries and declined aid.
Lt. Sue Stangl of the Seattle Fire Department said several people in the crowd were able to help medics as they responded to the multiple victims in the crash.
The driver of the car may have sustained minor injuries during the incident as well, officials said.
An investigation into the crash continues.