Greenpeace India vowed Thursday to overcome any attempt to silence its voice, with staff ready to work for free if funds run out, as it accused the government of waging a "malicious" campaign against it.

Executive director Samit Aich said the environmental pressure group was "hopeful" the Delhi High Court would uphold its petition challenging the government's move to freeze its accounts at a hearing next Tuesday.

But Aich warned that the group would not be silenced in its criticism of the right-wing government's record on the environment and that staff were even prepared to work for free if the case went against them.

"Today my staff sent me a very moving letter in which they promised to work without pay for one month," Aich said at a press conference.

"I hope it will not come to this and my staff and their families avoid this hardship. But if necessary, we are able to keep the basic functions of Greenpeace India running until the end of June."

Prime Minister Narendra Modi's government last month withdrew Greenpeace's foreign funding licence, citing violations of rules governing international financial transactions.

Following the ban Greenpeace said many donors in India were unable to deposit money into the group's local accounts, leading it to warn that it could go under within weeks.

Greenpeace has been at loggerheads with the government over claims about the environmental damage caused by India's heavy reliance on coal and the impact of deforestation and nuclear projects.

According to Indian media, a secret report by the main intelligence agency recently warned that delays to keynote developmental projects being sought by Greenpeace and other such organisations could knock up to three percentage points off the annual growth rate.

"We are being targeted by the home ministry in the most malicious manner," said Aich.

"It doesn't matter if you love us or hate us but you can't shut us down."

Speaking at the same press conference, Greenpeace campaigner Priya Pillai said Modi's government had been systematically dismantling regulations aimed at protecting the environment since it stormed to power last May.

Pillai, who was recently prevented from leaving India to address an conference in Britain, cited moves to overhaul laws on land acquisition and rights of tribal people as detrimental to the environment.

"Every progressive, strong law is being diluted, be it forest rights act or land acquisition laws," Pillai said.

The funding freeze on Greenpeace came shortly before the US Ford Foundation was hit with similar restrictions, prompting the American ambassador to New Delhi to warn of a "chilling effect" on democracy.