India's monsoon, dubbed an "economic lifeline", advanced along the southern coast Saturday after arriving nearly a week late as a heatwave baked the north, causing blackouts in the power-starved country.

As one of the world's top producers of rice, wheat and sugar, India relies heavily on the southwest monsoon which sweeps the subcontinent from June to September to water its crops.

"The India Meteorological Department (IMD) has declared the onset of the monsoon," the weather department said in a website notice Saturday.

The monsoon is being watched closely amid fears weak rains could further slow an economy grappling with its worst downturn since 1985 and pose a major challenge to the newly elected right-wing government of Premier Narendra Modi.

The meteorological department has predicted below average rains with a 23 percent chance of a "deficient monsoon".

Indians in southern Kerala state rejoiced as downpours eased scorching temperatures, but northern India was gripped by a heatwave with the mercury climbing as high as 48 degrees Celsius (118 degrees Fahrenheit).

In parts of the capital, New Delhi, people complained about up to 10-hour power cuts. Some staged protests against outages, a chronic problem in India where power generation infrastructure is ramshackle.

"Due to rising temperatures, demand for power has shot up," Delhi Lieutenant Governor Najeeb Jung told the Press Trust of India, warning, "There could be blackouts for another three to four days to cope with the crisis."

Rainfall is running at around 40 percent below average and the weather office said the rains may only advance to central India toward the end of June.

The monsoon is on a "sluggish march", IMD chief L.S. Rathore said.

India's monsoon arrived Friday, five days after it normally strikes, and some forecasters say risks of weak rains have been heightened by the El Nino effect, associated with a drier monsoon.

Just 40 percent of the nation's arable land is irrigated and poor rains can mean disaster for India's 235 millions farmers, many of them scraping a living.

Farming represents 14 percent of economic activity, down from 50 percent in the 1950s, but supports 700 million rural Indians who help drive consumer demand for cars to gold.

A good monsoon is seen as key to helping revive the economy growing at just under five percent, far below near double-digit expansion in boom years.

India has enjoyed normal or above-normal monsoons during the past few years but in 2009, the monsoon failed, shrivelling crops.

Rising food prices would also fuel headline inflation and keep interest rates high, discouraging growth-promoting investment, economists say.

"For this summer, inflation will still be driven by the weather," said investment house HSBC.