Armed police kicked in the door of a house in Cromwell Road after a major incident left three people seriously hurt, according to an eye-witness.
Shujah Saklain, a driving instructor with Easy as ABC Driving School saw armed officers crawling around Cromwell Road this morning, and a man has now been arrested.
Cambridgeshire police are expected to release a further statement later this morning (Tuesday).
Scenes of crime officers are at the scene and door-to-door enquiries are taking place.
Mr Saklain said: "I was on my way to work. I was going down Lincoln Road and saw armed police shooting down. I saw there was a situation in Cromwell Road and thought it was a drug raid, but then I saw an ambulance and knew it was something serious.
"It looked like a stand-off. The police said it was a serious situation.
"From what I could hear they were kicking the door in. At first they were standing back and shouting.
"I could see them charging in with some staying outside. I went back later and there were a lot more police there."
Police were called to the scene at 6.42am today and cordoned the road off. Ambulance crews and sniffer dogs were also in attendance.
Anyone with information regarding this incident should call police on 101 quoting CC-02072019-0061 or visit www.cambs.police.uk/report. Alternatively, contact Crimestoppers, anonymously, on 0800 555111 or via www.crimestoppers-uk.org.