"We call upon you to register an FIR in the matter. We are concerned that this being a high-profile case, witnesses or victim can be influenced or coerced and evidences can be tampered with," read the letter by 'Bharat Bachao Sangathan'.
The NGO also wrote to Delhi's Parliament Street police station under the jurisdiction of which Le Meridien hotel falls, where the incident allegedly took place on December 24, 2012.
NGO's president Vineet Ruia told PTI that a rally would be taken out on December 10, Human Rights Day, here demanding the arrest of Ganguly who is also the chairman of the West Bengal Human Rights Commission.
The NGO had earlier written to the Hare Street police station here and held a silent protest outside Ganguly's office, demanding that an FIR should be registered in the case.
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West Bengal Chief Minister Mamata Banerjee has twice written to President Pranab Mukherjee asking him to take appropriate action urgently against Justice Ganguly.
Ganguly has denied the charges and refused to comment on demands for his resignation from the WBHRC.