Plans for a £1.2bn electricity link between England and France could mean the loss of green spaces in Portsmouth, campaigners have said.
Aquind Ltd wants to lay a 238km (148 mile) cable between Lovedean in Hampshire and Normandy which would run through Portsmouth.
The firm said the two-way link could supply cheaper, greener electricity.
Protesters said its installation could destroy allotments, wildlife habitats and cause traffic disruption.
The planned interconnector could supply up to 5% of Great Britain's energy needs, according to Aquind.
Concerns have been raised about the route for the underground cables through Langstone Harbour, Farlington Playing Fields, Milton and Eastney Allotments and the Milton Locks Nature Reserve.
The final planning decision will be made by the government's planning inspectorate, rather than the local authority, because of the project's scale.
Portsmouth City Council has allocated £250,000 to opposing the scheme.
Council leader Gerald Vernon-Jackson said the planned route was "nuts".
"Traffic chaos across the city for a year is a price that is too high.
"But its the destruction of the allotments which has really galvanised people - they are completely right to be upset."
'Green energy vs green spaces'
Organiser of a protest at Eastney Beach, Ava Cadman said: "We've lost so many green spaces over the years in Portsmouth, I feel the disruption caused by this pipe is on the last few green spaces we've got.
"It is intrusive - the insects, birds, reptiles and sea life will all be affected.
"Green energy is great, but not green energy to lose your green spaces."
An Aquind spokesman insisted the allotment plots would not be "affected by the construction or operation of the project".
"A focus has been on considering the options included for the cable corridor and ensuring that the potential temporary impacts associated with the installation of the underground cables are minimised."
It aims to start delivering power in 2023 if the application to the Planning Inspectorate is approved.