Demonstrators torched a police vehicle in India on Monday as tensions boiled over after clashes involving protesting farmers killed at least nine people.

The incident on Sunday in Uttar Pradesh state was the deadliest in more than a year of protests by farmers in northern India against new agricultural reforms.

Farmers said that a convoy belonging to a government minister, his son and the state's deputy chief minister ran over and killed four people at a demonstration.

The minister said later that a driver lost control of his vehicle after being pelted by demonstrators.

Angry protesters set fire to several cars and at least five more people, four of them supporters of the ruling Bharatiya Janata Party (BJP), were killed.

On Monday, the protesters kept the bodies of the four dead farmers in glass cases for display around the protest site.  

Police banned gatherings, cut off mobile internet services, sent extra forces and detained several opposition figures on their way to the scene including Priyanka Gandhi from the Congress party.

In state capital Lucknow, dozens of police detained local Congress chief Akhilesh Yadav outside his home. 

Dozens of opposition supporters staged a protest in the city and set fire to at least one police vehicle, television pictures showed.

Protests organised by opposition parties also took place in New Delhi and Bangalore.

Agriculture has long been a political minefield and employs some two-thirds of India's 1.3 billion population.

Prime Minister Narendra Modi's government says the reforms will infuse much-needed energy and capital in the sector.

Farmers, many of whom have camped outside New Delhi for over a year, fear the changes will leave them at the mercy of big corporations.