The charismatic Swami Ramdev, who dons a saffron cloth thrown over his bare torso and in 2009 set up a retreat on the Firth of Clyde island of Little Cumbrae, runs a $40 million-a-year (24m) global yoga and health empire and has millions of followers: with 30 million viewers regularly tuning into his daily yoga TV show.
These followers are expected to rally behind him as he begins on Saturday a "fast-to-the-death" until the government agrees to pass a tough anti-corruption law and set up a task force for repatriating illegal funds held in foreign bank accounts by Indians.
"There will be over one crore (ten million] people who will fast," Ramdev told reporters at Delhi's airport after holding talks with four government ministers, rushed there by the prime minister to urge him to call off his fast.
"We want to get rid of corruption and injustices happening in institutions and we want to make things fair (in India]."
Singh has struggled to shake off a series of corruption scandals that have embroiled senior officials, including a $39 billion telecoms licensing scam, the biggest in India's history.
There is widespread public anger over the scandals, which have also damaged foreign investment and accelerated reforms aimed at opening up Asia's third-largest economy.
Ramdev's fast would be the second by a prominent figure to force the government to finalise a draft of the "Jan Lokpal" bill that gives an independent ombudsman police-like powers to prosecute ministers, bureaucrats and judges.
In April, veteran activist Anna Hazare, who is in his 70s, went on a hunger strike over the bill, triggering anti-corruption protests by thousands of people across the country.
He ended it five days later, after the government agreed to allow activists to take part in drafting the bill, and to introduce it in a parliamentary session due to start in July.
While Mr Hazare is widely respected - his campaign has drawn comparisons to Mahatma Gandhi's protests and hunger strikes that helped end British colonial rule, Mr Ramdev wields more clout and has vowed to launch a political party for the 2014 national elections to challenge Mr Singh's Congress.
Mr Ramdev, who does not disclose his age, sports a jet black ponytail and a very large black beard. He is also very wealthy and claims to cure cancer.
The yoga guru plans to hold his fast in a 250,000 sq-ft tent in the capital.In a last-ditch attempt, Mr Singh sent his trouble-shooter and political veteran, finance minister Pranab Mukherjee and three other ministers to welcome Mr Ramdev, who arrived on a private jet.
The prime minister is concerned the guru's fast will galvanise public opinion against the government.
"This is not a personal issue. We all agree with Ramdev that corruption is a big problem and that we are committed to tackling it with all the resources at our disposal," Singh said.
The government is resisting giving the Lokpal bill the teeth to probe corruption charges against the prime minister, members of parliament, top judges and other investigative agencies, saying this would harm the functioning of the state.
But many say this shows a lack of will to tackle corruption.
"In expressing its demands, the government has displayed how terribly out of touch it is with the nation," the influential Times of India said in an editorial yesterday.
Politicians fear that outrage over the scandals, made all the harder to stomach by rising food prices, may spark protests.
India ranked 78th on Transparency International's latest corruption index, lower than China. Beijing has dealt with corruption scandals with a rigour alien to India - execution being the ultimate price for many found guilty. Corruption has long been a part of daily life, but the current scandals in India are unprecedented