He has been on a public hunger strike since last Tuesday. An aide said he had lost 5kg, but was in good health.
Thousands of supporters are gathered at the vast Ram Lila Maidan grounds in the capital, Delhi, where Mr Hazare is conducting his fast.
Meanwhile, Prime Minister Manmohan Singh has said the government is open to talks with the 74-year-old activist.
Mr Hazare wants to force the government to strengthen an anti-corruption bill, which he regards as too weak.
He was released from Tihar jail on Friday after his arrest last Tuesday sparked mass protests across India.
On Thursday, he agreed to a police offer permitting him to go on hunger strike for 15 days.
He had previously vowed to remain in custody unless he was permitted to resume the protest which triggered his arrest. His campaign against graft has struck a chord with many Indians.Massive protests
Tens of thousands of supporters marched from the Delhi monument India Gate to the Ram Lila ground on Sunday.Continue reading the main story Following a hunger strike by Anna Hazare in April, the government agreed to draft the Jan Lokpal (Citizens' Ombudsman) bill. The final bill incorporates 34 of the 40 principles set out by Mr Hazare, but he and other activists have rejected itMr Hazare says the ombudsman should have the power to investigate the prime minister and senior judges. The government refuses to include them, saying their authority will be erodedMr Hazare wants the ombudsman to be able to investigate MPs accused of taking bribes to vote or ask questions in parliament. The government says such probes should be carried out by MPsDozens of protesters were detained after they demonstrated near the prime minister's house.
Thousands of people also joined a rally in support of Mr Hazare in Mumbai (Bombay) and protests were reported from the cities of Lucknow, Chandigarh, Ahmedabad and Surat.
This upsurge of support saw the authorities struggling to open a channel for communication with Mr Hazare.
On Saturday, PM Singh said the government was open to dialogue with Mr Hazare.
"There is a lot of scope for give and take... We are open to discussion and dialogue," Mr Singh said.
Reports said the government has roped in a senior bureaucrat from Maharashtra state, Umesh Chandra Sarangi, to negotiate with Mr Hazare.
"I had a few meetings with Mr Hazare over the Lokpal [ombudsman] bill. I have conveyed to him the views of the government. I am meeting him again on Monday," Mr Sarangi was quoted by the Times of India newspaper as saying.
Mr Hazare's supporters, however, said there had been no progress in resolving the standoff as the government had not made any fresh proposals.
Correspondents say Mr Hazare's fast has become a major embarrassment for the government and unless a compromise is found soon, it will further erode the credibility of the government.String of scandals
Mr Hazare says the government's proposed anti-corruption bill is weak and wants his own tougher version to be passed by parliament.
Prime Minister Manmohan Singh has accused Mr Hazare of trying to circumvent democracy by demanding the overhaul of the bill.
But the spontaneous outbreak of public support has become a matter of deep concern for Mr Singh's administration.
India has recently been hit by a string of high-profile corruption scandals which critics say is evidence of a pervasive culture of corruption in Mr Singh's administration.
A recent survey said corruption in Asia's third largest economy had cost billions of dollars and threatened to derail growth.