Hundreds of thousands of coal miners in India have downed tools, threatening power blackouts and challenging the country's new reformist prime minister.

The planned five-day strike has entered its second day as negotiations with the government over plans to privatize the industry remain at an impasse. The strike is the biggest of its kind in nearly four decades.

Coal accounts for more than half of India's energy needs.

State-run Coal India - the organization unions are striking against - provides 80% of India's coal output.

Coal India spokesperson Vijay Sagar told CNN the corporation produced just two-fifths of its usual capacity on Tuesday, the first day of the strike.

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Indian media has reported some power plants have just four days of coal stocks left. There have been no reports of an increased number of power outages so far.

The issue at stake is what unions see as an impending government move to privatize India's coal sector. Prime Minister Narendra Modi was elected last year and the plans are part of wider reforms to spur economic growth.

A spokesman for one of the striking unions, the Bharatiya Mazdoor Sangh, said privatization will lead to job losses.

"We want job security," said Virjesh Upadhyay, "Once the sector becomes private a lot of people will be laid off. Their wages will decrease."

Many economists say privatization is necessary to boost efficiency.

"Privatization of the coal sector would be a positive reform," said Dipesh Dipu, energy analyst at Jenissi Management Consultants.

"There will be more production of coal and this will help India in many ways."

S. Q. Zama, the secretary-general of India's National Mineworkers' Federation, told CNN the protest could extend beyond five days if there was "no political solution" to their demands.

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CNN's Kunal Sehgal and Sania Farooqui contributed to this report.