Five Indian farmers were shot dead on Tuesday at a protest in the central state of Madhya Pradesh, according to news reports, marking an escalation of violence as a rural strike demanding debt relief spread.

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The outburst of discontent in India's heartland farming states of Madhya Pradesh and neighboring Maharashtra poses a challenge for Prime Minister Narendra Modi, who has promised to double farmers' incomes over the next five years.

The farmers were shot during a protest in the central city of Mandsaur. The spokesman of one farming union said police had fired on the protesters.

"Police started firing to disperse the crowd. Farmers were not carrying weapons," said Gajendra Tokas of the Rashtriya Kisan Mazdoor Sangh, or National Farm Workers' Union, which has called a state-wide strike on Wednesday.

The state's home minister, Bhupendra Singh, said police had not shot at the protesters, adding: "Anti-social elements in the crowd fired bullets."

Farmers last week started a strike in the state and in western Maharashtra, dumping vegetables and milk on the roads to demand billions of dollars in debt forgiveness and better prices for produce.