Bridport Town Council has proposed allowing 100 cars to park on a grass area of Asker Meadows during the summer weekends and festivals.
Ged Duncan, of the Friends of Asker Meadows, said: "It's a nature reserve, not a car park."
The council said the field would only be used on a limited number of days.
More than 700 people have signed a petition opposing the plans to allow parking on the field, part of the 19-acre local nature reserve alongside the River Asker.
Mr Duncan said: "We can't see how having car parking on the site fits in with the objectives of wildlife, recreation and education.
"We understand the need for business and tourism to enable access for people to visit Bridport but we need to look for sustainable ways of doing that - rather than seeing a space and saying 'lets stick some cars on that'."
Bridport Town Council said the area being considered was a show field currently used for events and the rest of Asker Meadows would not be affected.
The plans are currently open to public consultation.
Plans to allow car parking on a Dorset nature reserve would "spoil" the visitor attraction, campaigners have claimed.