A senior campaigner for the environmental group Greenpeace was stopped on Sunday morning from boarding a flight to London where she was to brief British MPs on the rights of forest-dwelling communities affected by coal mining in India.
A statement from Greenpeace said Pillai had a valid six-month business visa to visit London, where she was scheduled to address British parliamentarians on 14 January.
She was invited by the British MPs to talk about her campaigning with local communities in Mahan, Madhya Pradesh, where a proposed coal mining project led by Essar, a London-based but India-focussed company, threatens to uproot the lives and livelihoods of the forest community, to say nothing of the general damage to the environment.
The incident comes a few months after the blocking of funds to Greenpeace by the ministry of home affairs.
The NGO has reacted with shock. Pillai expressed disappointment at the government ''riding roughshod over people working to protect democratic rights in the country''.
She added that her right to freedom of movement has been infringed and she is being treated her like a criminal.
Pillai was told by the airport authorities that she is banned from leaving India, even though she has no criminal convictions. Her passport has been stamped 'Offload', according to a Greenpeace statement.
The immigration officials told her they were following orders from the union government.
Greenpeace India's executive director Samit Aich said in the statement, ''The government's intentions are clear. They are trying to intimidate and bully Greenpeace and its employees ... I would only reiterate that such acts make us more resolute to continue our campaigns to protect Indian people and India's environment. We will not shy away from asking tough questions to the highest authorities.
''Such systematic excesses by the government are a shame and cause of worry for the Indian civil society. At a time when the whole world is making a strong pitch to safeguard freedom of speech and democratic rights, this action by the world's largest democracy is problematic.''
Greenpeace India has written to the home and external affairs ministries and to the Airport Authority of India asking the government to explain the legal basis of the ban on Pillai leaving the country.