Police have cordoned off a street in East Oxford as well as the front of a nearby bar in Cowley Road following reports of a stabbing.
Officers shut off Divinity Road at about 8pm and put up police tape in the area.
Cowley Road Carnival celebrations have been taking place throughout the day.
A Thames Valley Police officer at the scene told a passer-by there had been a stabbing but the force has not yet provided any further information.
South Central Ambulance Service staff were called at 7.54pm but a spokeswoman would not comment on the nature of the incident.
Divinity Road resident Martin Stott said he tried to visit the Co-op on his way home but was prevented from doing so by a policeman.
He told the Oxford Mail: "I was walking back from my allotment at about 8pm and tried to go into a shop to buy some fruit, but a policeman ran over and told me I couldn’t walk there because there had been an incident.
"There are a lot of police and they are all outside the Co-op.
"The road is closed from top to bottom.
"When I first arrived they had not closed it off but they quickly got out the tape and were making cars reverse back up their road."
Prakash Sharma, manager of the nearby Sherpa napalese cafe, said he saw the ambulance arrive.
He said: "The police will not tell us anything, except that a serious incident has happened."