Indian Prime Minister Manmohan Singh has told parliament that the hunger strike by anti-corruption activist Anna Hazare is "totally misconceived".
He said the 74-year-old was trying to circumvent democracy by demanding the overhaul of an anti-corruption bill.
Mr Hazare spent the night inside Tihar jail in Delhi after rejecting an offer by the authorities to release him.
He has said he will remain there unless he can resume the public protest stopped by his arrest on Tuesday.
Protests backing his campaign have been gathering momentum across the country.
Thousands of people have gathered outside Tihar jail, many carrying the Indian flag and shouting "Down with corruption" and "Hail Mother India".
There were also reports of large demonstrations at India Gate in Delhi, in Mumbai, Chandigarh, Hyderabad, Bangalore, Ahmedabad, Amritsar, Bhubaneshwar and in the north-eastern state of Assam.
Supreme Court lawyers have announced they will march to show their support for Mr Hazare, while auto-rickshaw drivers have gone on strike. (more)