An Indian anti-corruption activist walked out of jail Friday, hailed by a huge crowd, to carry on his fast after the government caved in to mass protests across the country.

After three days in jail, Anna Hazare will begin a 15-day hunger strike. Arrested earlier in the week and then ordered released, had refused to leave jail until he was allowed to fast publicly.

Hazare's exit was broadcast live across India as thousands of supporters thronged outside the jail in a deafening roar of celebration.

Just past the gates he addressed the crowd, raising his hand to the air and shouting Victory to Mother India before slowly winding his way in a truck decorated with flags through massive crowds.

Many supporters had been there overnight and some had offered prayers to the Hindu gods. Others chanted the whole country is Anna.

A top Hazare aide said he would first go to a memorial to Mahatma Gandhi and later travel to open grounds in central Delhi, where supporters were making final preparations for his fast.

We have not seen this kind of thing in the last 60 years in India, said S.K. Sharma, 48, a company executive CEO, outside the jail as he waited for Hazare. If this carries on in this way for the next four days, you will see a new changed India.

A fumbling official response has seen the Congress party-led government face one of the most serious protest movements since the 1970s, just the latest setback for Prime Minister Manmohan Singh's second term that has paralyzed policy making and economic reforms.

A medical team is on standby to monitor Hazare's health and a sharp deterioration in his condition could further worsen the crisis for the government, although his supporters say it is not a fast to the death.

A weak political opposition means that the government should still survive the crisis, but it could further dim the prospect for economic reforms and hurt the Congress party in key state polls in 2012 that will pave the way for a general election in 2014.

The protests across cities in India, helped spread by social networks, have not only rocked the ruling Congress party, they have sent shockwaves through the political class.

Students, lawyers, teachers, executives and civil servants have taken to streets in cities and remote villages stretching to the southern end of the country.

In the financial capital, Mumbai, the city's iconic lunch carriers, known as Dabbawallas, were reportedly set to go on strike in sympathy with Hazare.

One Facebook fan page for Hazare has more than 300,000 followers, while the India Against Corruption page on Facebook has more than 370,000 followers where links and messages of support are posted. Several Twitter accounts have been set up by supporters to send out messages of where and when to protest.

Singh, 78, who is widely criticized as out of touch, dismissed the fast by Hazare as totally misconceived and undermining the parliamentary democracy.

Hazare, a long-time social activist who follows the tactics of Mahatma Gandhi, became the unlikely thorn in the side of the ruling coalition when he went on hunger strike in April. He called off that fast after the government promised to introduce a bill creating an anti-corruption ombudsman.

The so-called Lokpal legislation was presented in early August, but activists slammed the draft version as toothless because the prime minister and judges were exempt from probes.

(Editing by John Chalmers)