The demonstrators were responding to a call from Hazare from the jail where he has been incarcerated since his arrest on Tuesday morning.
Hazare was detained as he prepared to begin an indefinite hunger strike to protest against what he sees as diluted anti-corruption laws recently introduced in India's parliament.
In the face of mounting pressure from the street, orders were given for Hazare's release on Tuesday evening, but he refused to leave prison without a guarantee that he could proceed with his hunger strike in one of the capital's public parks.
Indian Prime Minister Manmohan Singh told parliament on Wednesday that Hazare's action was a challenge to the authority of lawmakers, as legislation was their prerogative alone.
He was repeatedly heckled by opposition parliamentarians.