Hundreds of Bangladeshis began a four-day march Thursday from the capital to the Sundarbans in a last-ditch protest against plans to build a coal-fired power plant near the World Heritage-listed forest.

Preliminary work has already begun under an Indian-Bangladesh joint venture on the massive plant which will provide much needed electricity to the impoverished country when it becomes operational in 2018.

But organisers hope the 250-kilometre (155-mile) long march will force the Bangladesh government to scrap the plant which they say will harm endangered Bengal tigers and other animals living in the mangroves.

"It's now or never. We can't allow this power plant to destroy the world's largest mangrove forest," march organiser Ruhin Hossain told AFP.

Nearly 1,000 students, green activists and others rallied in Dhaka, shouting slogans and carrying flags of left-wing political and labour groups before starting the march which they hope will draw thousands of others along the route.

"The Sundarbans is one of the pristine beauties of the world," Hossain told AFP.

"In the past it has saved us from cyclones, flash floods and it's our biggest protection against tsunamis," he said of the ecologically fragile forest which acts as a natural buffer against extreme weather.

Experts said the 1,320-megawatt plant is being built just four kilometres from the UNESCO-declared World Heritage section of the vast forest which straddles Bangladesh and India.

India and Bangladesh signed a deal in 2010 to jointly develop the $1.7-billion power plant.

Bangladesh's forestry chief has said authorities have "adopted an environment management plan to mitigate any possible negative impact on the forest".

But some experts fear the plant will dump tonnes of coal waste into the 10,000-square kilometre (3,800-square mile) forest, already suffering from over population and pollution.

"Coal-based power plants have the most emissions and we shouldn't forget incidents like the oil spill in a Sundarbans river," Nurul Amin, an environmental science professor at Bangladesh's North-South University, told AFP.

In 2014 a boat carrying oil spilled thousands of gallons into the forest, with a lack of formal relief efforts forcing locals to use spoons, pots and sponges to clean up the sludge.