Septuagenarian Anna Hazare's campaign draws on widespread public anger over a spate of corruption scandals that have emerged in Prime Minister Manmohan Singh's second term, tarnishing the government's image and spooking investors.
The biggest scandal involves charges that rule violations during a 2007-08 grant of telecoms licences may have cost the government as much as $39 billion in lost revenue. The then-telecoms minister has been forced to resign and has been arrested.
Hazare has been fasting since Tuesday on a street-side platform in the shadow of an 18th century astronomical observatory in New Delhi, demanding the government enact the Jan Lokpal bill, a stringent anti-graft law.
Crowds in the capital and other cities across the country have swelled each day, with word being spread through extensive media coverage, text messages and social networking sites like Facebook and Twitter.
On Friday, the thousands gathered before Hazare shouted anti-graft and anti-government slogans, vowing not to yield until the Congress-led coalition government agrees to their demands.
Protesters held up banners that read: "It's the talk on the streets, my leader is a thief". Badges read: "Manmohan Singh want my vote? Support the Jan Lokpal Bill."
Hazare has demanded that members of civil society sit with ministers to draft the bill, which would give an independent ombudsman police-like powers to prosecute ministers, bureaucrats and judges.
"The people's voice has reached your (government's) ears. If you remain deaf, the people will teach you a lesson," Hazare told his supporters, to loud cheers.