Ain o Salish Kendra (ASK), Online Journalist Association of Bangladesh (OJAB) and several other organisations have also condemned his arrest.
The senior journalist was picked up by detectives on Sunday over a Facebook status.
Within hours, Sikdar was accused of libel in a case filed under the Information Communication and Technology Act at Faridpur.
Ganajagaran Mancha, the mass platform seeking maximum penalty for 1971 war criminals, took out a march at Shahbagh’s Prajanma Chattar at 5pm Monday.
Spokesperson Imran H Sarker announced the programme on Facebook at noon.
Meanwhile, the journalist was produced in a Faridpur court in the afternoon and ordered into prison. Police sought to question him in custody for 10 days.
The case against Sarkar was filed at Kotwali Police Station at 11pm Sunday for ‘damaging the good name’ of LGRD Minister Khandker Mosharraf Hossain.