Human rights activists have called on India to reform or repeal laws that threaten free expression in the world's largest democracy and muzzle charities such as environmental group Greenpeace.

Some laws are not only silencing marginal voices but are also fuelling graft, a report by PEN International, a London-based group of writers promoting freedom of speech, and the International Human Rights Program at the University of Toronto said.

"We hope this report will help the government and courts further review laws that silence dissent and move forward with the necessary repeals and reforms," said Marian Botsford Fraser, chair of PEN International's Writers in Prison Committee.

The report, based on more than 30 interviews with activists and writers, said that a law on foreign funding to charities was being used to lodge complaints against non-profits that do not agree with government policy.

Law ministry officials were not immediately available for comment.

Since Prime Minister Narendra Modi swept to power almost a year ago, his right-wing nationalist government has tightened surveillance on foreign-funded charities.

The Indian branch of Greenpeace, which has led campaigns against genetically modified crops, coal mining and nuclear power projects, said this month it was facing shutdown after authorities blocked foreign funding and froze its bank accounts.

In April, the government also cancelled the licenses of almost 9,000 charities saying that some had violated the law by not disclosing details of their donations.

The charities say they are being targeted selectively by authorities as part of a crackdown.

"We can't be seen as influencing public policy through public campaigns," the head of an Indian charity, who asked to remain anonymous, said in the report released this week.

As a result, the non-profit avoided or downplayed discussions about religious issues and human rights reporting from some disputed areas in the northeast of the country.

Other laws in India such as those on blasphemy, obscenity and sedition have allowed groups or individuals to silence one another, said the report.

For example, Aseem Trivedi, a prominent anti-corruption campaigner, was arrested in Mumbai in 2012 under sedition laws after a complaint was lodged against him for publishing a series of cartoons that satirised India's national symbols.

In 2014, the owner of business park in Jaipur was charged with obscenity for displaying paintings that depicted nude women on the roof of his gallery, after a citizen claimed they were causing him "mental agony".

"The most frightening thing is that any mad coot can go and lodge a complaint against you. It's a serious amount of harassment," Arundhati Roy, one of India's most famous writers, who was herself charged over comments she made on the disputed region of Kashmir, was quoted as saying.