Even though 62 of the persons arrested were released, Roy, along with six others, was sent to police custody.
The arrest has raised international concern as renowned intellectuals such as Noam Chomsky and a dozen other scientists and public figures have requested PM Manmohan Singh to release him.
The anti-eviction movement, in a place called Nonadanga in Kolkata, started after the state government demolished some shanties on March 30.
With the participation and guidance of some activists from the Mazdoor Kranti Parishad, a leftist trade union, and members of various mass organisations, those evicted took out a protest rally on April 4. Roy participated in the protest.
The scientists who have written to the Prime Minister have also expressed concern at the treatment meted out to Roy in custody.