Thousands of protesters gathered in the Bangladesh capital Saturday, demanding the government scrap a massive coal-fired power plant they say will destroy the world's largest mangrove forest.

Slogan-shouting activists travelled from all over the country to join the demonstration at the Shaheed Minar memorial in Dhaka.

Campaigners have been protesting for the last three years against the under-construction plant which is 14 kilometres (nine miles) north of Sundarbans forest, part of which is a UNESCO world heritage site.

"We can build as many power plants as possible. But we can't create another Sundarbans if it is destroyed by the Rampal power plant. All the rare tigers, dolphins and other animals will be destroyed," said Saddam Hossain, 21, from the northern city of Bogra.

"It is a unique forest and one of its kind in the world. But by building the power plant, the government is writing its obituary," said student Mashuk Helal Onik.

Organisers said more than 20,000 people had joined the protest and were expecting up to 100,000 as the day progressed.

Police were reluctant to give a figure but said the number would be lower than the organisers' estimate.

Last month UNESCO urged Bangladesh to halt construction of the plant.

It said there was a high chance pollution from the plant would "irreversibly damage" the Sundarbans, which straddles the border of India and Bangladesh and provides a barrier against storm surges and cyclones that have killed thousands of people in impoverished coastal villages.

Bangladeshi Prime Minister Sheikh Hasina has defended the project and rejected concerns about it as politically motivated. She said the plant was needed to provide power to the impoverished south.