A protest campaign by the ethnic Gujjar community to demand more government benefits has degenerated into bloodshed and lawlessness, and police said they would act to stop the unrest.

"Anyone seen indulging in arson and rioting will be shot," local police inspector-general Ajit Singh said.

The unrest in the western state of Rajasthan began Tuesday at a protest attended by thousands of Gujjars, who are the dominant community in nine of the state's 32 districts but make up only five percent of the state population.

The community continues to block the state's main highway to New Delhi, and several train services have been cancelled. The unrest has prompted the US embassy in India to issue a travel warning to its citizens.

The violence threatened to snowball into an inter-caste clash Friday as the Meena community, primarily farmers and traders, warned it would take action if the state accorded favours to the Gujjars, who are nomadic shepherds.

The Meenas, comprising 15 percent of Rajasthan's population, have better access to education compared to the Gujjars and are better represented in the state's law enforcement agencies.

Talks between the state government and Gujjar leaders have failed to yield any result so far.

The trouble spread Friday to neighbouring Haryana state, where police fired in the air to chase away Gujjars blocking another highway to the Indian capital.

In an army show of strength, heavily-armed troops rode military trucks through deserted highways in the area, officials said.

Three more people died from bullet wounds after being fired on by police in the unrest, taking the death toll to 20. At least two police officers are among the dead.