A member of the public called officers after a number of tents were found on Monday on land off Falkland Road, Chandler's Ford.
Police also said a fire was reported to have been started in the area.
Three men, aged 19, 20 and 26, and a 17-year-old girl, all from the local area, have been arrested.
A police spokeswoman said they had been held on suspicion of sexual offences but would not give any more details.
She added it appeared to be an isolated incident.
Three men and a teenage girl have been arrested on suspicion of sexual offences after an area of woodland was sealed off by police in Hampshire.