LEGAL action is being launched against an unauthorised encampment.
At least five caravans and seven vehicles arrived on Stockheath Common, Leigh Park, on Thursday.
The land, which is bordered by Riders Lane, Stockheath Lane and Purbrook Way, is owned by Portsmouth City Council.
A spokeswoman for the authority said it completed a welfare assessment of the encampment and is now taking legal action to remove it.
Thousands attend Stockheath Common every year for a fireworks display, car boot sales and other community activities.