Up to 75 campaigners brought traffic to a standstill as they marched through Bedford town centre in a protest over plans to build a freshwater aquarium.
The centre will be a world leader in aquatic research
The bio-domed research complex and aquarium, four times the size of the Eden Project, would be at Stewartby.
Bedford Animal Action said it opposes The National Institute for Research into Aquatic Habitats project on the grounds of animal cruelty.
The East of England Development Agency says it will boost the local economy.
The demonstration held on Saturday included members of the Green Party, the Captive Animals Protection Society and SPEAK.
Campaigners are also concerned that the project will damage the local environment.
The government has already given the go-ahead for the East of England Development Agency (EEDA) to provide loans for the £250m project.
The money would help the National Institute for Research into Aquatic Habitats (NIRAH) develop a business plan and seek planning permission.
The 40-hectare complex, in an old brickworks, is the brainchild of an international team of biologists and conservationists.
It would be designed by Nicholas Grimshaw, who was responsible for the Eden Project in Cornwall, and would be stocked with tropical trees and plants and populated by thousands of species of freshwater fish, amphibians and reptiles.
George Kieffer, deputy chair at the East of England Development Agency and one of the leaders on the Bedfordshire NIRAH Consortium, said the centre would generate new jobs and would have a significant impact on the economy of Bedfordshire and the East of England.
It is hoped the centre, funded by the revenue from scientific research and development as well as visitor income, would boost understanding of the Earth's freshwater system and the animals that rely on it.
If plans are approved the centre could be running by 2010.