Paramedics were called out to Abington Street in the early hours of yesterday morning after reports of a "trail of blood" leading from Abington Square to the nearby Burger King.
A 35-year-old man was left with serious but non-life-threatening injuries and was taken to Northampton General Hospital after what was believed to have been an assault.
A witness, who did not wish to be named, said: "Two paramedics were looking after a man who looked like he had a nasty cut to his head. It looked like he had been in a fight. They were all standing, though He wasn't on the ground.
"The end of Abington Street was taped off by police and two police cars were parked."
The incident reportedly took place around 6.30am, but the police cordon was taken down by 8.30am before many shops were opened.
A police spokeswoman said the force was called out at 6.25am on Sunday to reports of an assault in Northampton town centre.
The 35-year-old victim is currently in a stable condition.
Northamptonshire Police is calling for witnesses, or anyone with information to contact the non-emergency number on 101.