A popular anti-corruption crusader held a day-long fast in the Indian capital on Sunday, urging supporters to court arrest and fill up prisons nationwide if the beleaguered government failed to enact a tougher law to fight graft.

The spectre of renewed protests led by the bespectacled Gandhian campaigner Anna Hazare come at a difficult time for Prime Minister Singh, who is increasingly seen as a lame duck at the half-way stage of his second five-year term.

Last week, at risk of losing key partners from the ruling coalition, Singh backtracked on a flagship reform to allow foreign investment in supermarkets.

Dressed in a crisp white kurta smock and cap, Hazare told hundreds of supporters that a draft legislation setting up a Lokpal (ombudsman) to investigate wrong-doing in government was a betrayal.

Fill up the jails if that is what's needed for a strong Lokpal (ombudsman) bill. We'll ensure all the jails are filled, we won't leave a single one empty, he said to loud cheers at Jantar Mantar observatory, a traditional meeting point for political dissenters in Delhi.

Hazare has caught the imagination of a swelling middle class in India who are angry at the government's inability to crack down on rampant corruption after multi-billion dollar scams related to telecoms and the 2010 Commonwealth games came to light.

In August, he was arrested for three days just hours before he was due to begin a fast to the death with similar demands and was dismissed as an anarchist by Prime Minister Singh. He broke his fast, which he started in jail, after the government assured him of meeting his demands.

The turnout at Sunday's protest was seen as a barometer of support for the 74-year-old, who has threatened to campaign against the already troubled government in elections in India's most populous state early next year.

Hundreds of people with the Indian flag in hand and wearing I am Anna caps, sang patriotic songs as Hazare reached the venue of the strike on Sunday after praying at independence hero Mahatma Gandhi's memorial.

Our government takes our money through taxes, through bribes and uses that again to buy votes. How can my grandchildren later say that they are proud to be Indians? said 56-year-old H.S. Kapoor at the protest site.

Once strongly supportive, the Indian media has taken a more critical view of Hazare's Gandhian credentials in recent weeks after he appeared to support public flogging of alcoholics, said corrupt politicians should be hung and suggested an elderly politician who was slapped by a protestor had it coming.

PLANS MORE FASTS

The anti-graft bill is likely to be discussed next week by lawmakers and Hazare is adamant it must be passed in the form he wants before parliament closes for the year on December 21.

He has threatened a 10-day hunger strike from December27 if the government failed to do so.

The main opposition Bharatiya Janata Party, which is seeking to pile on the pressure on the ruling coalition, backed Hazare's call for stronger legislation against corruption.

There is no two ways about this that the country needs a strong and effective Lokpal bill, Arun Jaitley, the BJP's leader in the upper house of parliament, told the gathering.

The BJP and other political parties are demanding that the prime minister's office be brought under the ambit of the ombudsman. It is not yet clear whether the draft bill addresses that demand.

Brinda Karat, a leader of the Communist Party of India (Marxist), said the ombudsman must also have the powers to investigate big corporate houses, many of whom have been named in a series of corruption scandals.

The ruling Congress party did not take part in Sunday's debate, which was convened by Hazare. It said it was premature to debate the bill when parliament was seized of the matter.

Several India states go to the polls next year including the country's most important political region, Uttar Pradesh, sharpening the political jousting ahead of the election.

(Additional reporting and writing by Anurag Kotoky; Editing by Frank Jack Daniel and Sanjeev Miglani)