Violence exploded at Leigh Park as a gang of weapon-yielding youths clashed with young fairground workers.
Seventeen people were arrested after police reinforcements were drafted in to end sporadic outbreaks of violence during the day.
Trouble erupted around the fair at Stockheath Common.
Between 10 and 15 youths were moved on after fighting which left one youth with a cut ear and another with facial injuries.
Both needed treatment at Queen Alexandra Hospital, Cosham.
Police units were brought from Havant, Cosham and Portsmouth in an attempt to quell violence which flared again in the evening.
Officers arrived at Blendworth Crescent to find the troublemakers dispersing – leaving sticks on the ground behind them.
They were believed to have been used as weapons during the fighting, a spokesman for Havant police said.
Alcohol was not thought to lie behind the cause of disturbance.
The 17 arrests were made about 45 minutes later with youths aged up to 19 all being reported for alleged public order offences.