Around six caravans were seen at Fraser Range, in Eastney, Portsmouth.
Police said they received a report at around 2.45pm today.
Defence firm QinetiQ owns the site. It is in talks with the city council over its future.
Eastney ward councillor Luke Stubbs: ‘Fraser Range has been unused for many years now and that has inevitably resulted in some problems occurring.
‘I’d like to see QinetiQ do more to secure that site.
‘Now that travellers are however there, it’s important that the police, council and others work together to move them on as soon as practicable.’