Travellers drove on to Harpenden Common on last night (10) with reports of at least 16 caravans parked there at one time.
The common, which is maintained by Harpenden town council, is a designated county wildlife site.
The caravans have been parked on the Hayfield, a part of the common which has a diverse range of wildflowers and butterflies.
Police have been to the site and are said to be working in partnership with the landowners to try and resolve the issue.
On the Harpenden Parents Network Facebook Group, former town Mayor, Cllr Mary Maynard, said around 16 vans had been parked on the Hayfield but that number had later dropped to six or seven.
She said that because it was a county protected wildlife site it was under a higher level stewardship scheme governed by Natural England.
Also, as it was a conservation area, it was protected by town council by-laws.
No-one from Harpenden town council was available to make any comment on the situation before the Herts Advertiser deadline today.
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