Prime Minister Narendra Modi wielded a broom in a New Delhi slum on Thursday as he pledged to sweep away India's reputation for poor public hygiene and rudimentary sanitation.

Hours after flying home from Washington, the energetic premier rolled up his sleeves and picked up a brush to launch a 'Clean India' campaign on a public holiday which celebrates independence icon Mahatma Gandhi's life.

"Gandhi gave us the slogan: 'Quit India' and people came together to win our country's freedom (from British colonial rule)... but his other dream of 'Clean India' is still incomplete,", Modi said in a televised address.

The 64-year-old Modi, who came to power in May, has made public health one of the main priorities of his new administration and has already promised to ensure all schools have separate toilets for girls and boys.

A new report by the UN children's fund UNICEF estimates that almost 594 million -- or nearly 50 percent of India's population -- defecate in the open, massively increasing the risk of disease in densely-populated areas.

Modi said that the responsibility for public cleanliness did not rest solely with sanitation workers but required a change of mindset in a country which recently celebrated a successful space mission to Mars.

"It ('Clean India') can be achieved, it can happen. If we can reach Mars, then why can't we clean our neighbourhoods?", he added.

As part of the campaign launch, ministers and civil servants were expected to take part in a mass clean-up on Thursday of government buildings, many of which stink of stale urine and are littered with rubbish.