NEW DELHI, Oct 4 (Reuters) - Farmers in India vowed on Monday to intensify their months-long protest against laws aimed at liberalising agriculture as tension flared a day after eight people were killed in clashes between growers and ruling party supporters. Four of the eight were killed when a car linked to a senior ruling party official crashed into protesters in Uttar Pradesh state, protest leaders said. read more Police said they were investigating the crash and had registered a case against 13 people including a son of a minister of state in the interior ministry, Ajay Kumar Mishra, An alliance of protest groups called in a petition to President Ram Nath Kovind on Monday for a court-supervised investigation of the violence. "We"ll intensify our agitation in Uttar Pradesh and other parts of the country to highlight the plight of innocent farmers," Dharmendra Malik, a senior leader of the Bharatiya Kisan Union, told Reuters. "The government might try to discredit the 10-month-long movement, but we will remain peaceful in our struggle," Malik said. The legislation the farmers object to, introduced in September last year, deregulates the sector, allowing farmers to sell produce to buyers beyond government-regulated wholesale markets, where growers are assured of a minimum price. Small farmers say the changes make them vulnerable to competition from big business, and that they could eventually lose price supports for staples such as wheat and rice. The government says reform of the sector, which accounts for about 15% of the $2.7 trillion economy, mean new opportunities and better prices for farmers. Farmers say the car that crashed into the protesters was owned by a son of junior home minister Mishra. Mishra said his son was not present at the incident but a car driven by "our driver" lost control and hit the farmers after they threw stones at the car and attacked it with sticks and sword. The son, Ashish Mishra, also denied being present. Police in Uttar Pradesh, which is ruled by Prime Minister Narendra Modi"s party, said that the son was one of the 13 people against whom a case had been registered. India"s longest-running farm protests pose a risk for the re-election prospects of Modi"s Bharatiya Janata Party when Uttar Pradesh, India"s most populous state, goes to the polls early next year. Since Sunday, several opposition politicians have tried to reach the Lakhimpur Kheri sugar cane-growing district where the violence took place. "For many months, farmers have been speaking out, saying that they are being wronged but the government is not willing to listen," said Congress party leader Priyanka Gandhi Vadra.