Two people have been stabbed, a third found dead in a flat and three left for dead after being hit by a speeding van.
The string of attacks in Nottingham centre has forced the city into lockdown.
A 31-year-old man has been arrested on suspicion of murder.
Nottinghamshire Police said two people were found dead in Ilkeston Road at around 4am on Tuesday.
The body of another victim was discovered in Magdala Road.
Ninety minutes later three more people were hurt when someone tried to run them over in a van in Milton Street.
A witness told the BBC he saw a young man and young woman being stabbed on Ilkeston Road.
The man, who did not give his name, said he heard “awful, blood-curdling screams”.
He said he looked out his window to see a “black guy dressed all in black with a hood and rucksack grappling with some people”.
He told the broadcaster: “She was screaming: ‘Help!’
“I just wish I’d shouted something out of the window to unnerve the assailant.
“I saw him stab the lad first and then the woman.
“It was repeated stabbing – four or five times.
“The lad collapsed in the middle of the road.
“The girl stumbled towards a house and didn’t move.
“The next minute she had disappeared down the side of a house, and that’s where they found her.
“I’d say it all happened within five or six minutes.
“The attacker then just walked off up Ilkeston Road towards town, as calm as anything.”
Kane Brady, a student at the University of Nottingham, told GB News he saw a knife being taken from a white van after a man was arrested outside his house.
Mr Brady said:
“We woke up to shouts of ‘armed police’ and what… sounded like some very loud noises, what sounded like gunshots – it was that loud.
“I looked out the bedroom window and saw Tasers.
“I saw a man being dragged out (of the van) and pinned to the floor.”
Witness, Lynn Haggitt, told Channel 4 News she saw a van hitting two people at around 5.30am near the Theatre Royal.
She said: “I saw a van pull up at the side of me.
“It was white, all white.
“There was a police car behind it, coming up slowly, no flashing lights.
“The man in the driver’s seat looked in his mirror and saw the police car behind him.
“The white van then drove up to the corner of the street and went into people.
“The lady ended up on the kerb, and then he backed up the white van and he went… speeded up Parliament Street with the police cars following him.”
Nottinghamshire Chief Constable Kate Meynell said:
“This is a horrific and tragic incident which has claimed the lives of three people.
“We believe these three incidents are all linked, and we have a man in custody.
“This investigation is at its early stages and a team of detectives is working to establish exactly what has happened.
“We ask the public to be patient while inquiries continue.
“At this time, several roads in the city will remain closed as this investigation progresses.”
The force has not yet commented on whether the attacks are being treated as terrorism.
Prime Minister Rishi Sunak thanked the emergency services for their response to the “shocking incident”.
He added: “My thoughts are with those injured, and the family and loved ones of those who have lost their lives.”
