The protestors, led by Sanchaya leader Dinesh Holla, gathered near Nehru Maidan, the venue of the Republic Day celebration and staged a black flag demonstration.
They were detained and removedby police.
The activists had been protesting against Rai for the last few months for his open support to the river diversion project.
Another group of six activists also staged a protest near the gate of the Nehru Maidan after the function was over.
All the protestors were later released, police said.