Crowds in India set a train coach on fire and blocked railway tracks and roads in protests on Thursday against a new military recruitment system, police said.

Prime Minister Narendra Modi's government this week announced an overhaul of recruitment for India's 1.38 million-strong armed forces, looking to bring down the average age of personnel and reduce pension expenditure.

In eastern India's Bihar state, where protests have flared in around a dozen locations, thousands gathered in Nawada city to demonstrate against the new recruitment system, police official Gaurav Mangla said.

“They torched a BJP office, torched tyres in three prominent areas of the city, damaged a bus and many private vehicles,” Mangla told Reuters.

Protesters also attacked railway property across Bihar, settling alight coaches in at least two locations, damaging train tracks and vandalising a station, according to officials and a railways statement.

Police said protests also took place in northern Haryana state and western Rajasthan — both traditional recruiting areas for the Indian military.

The new system, called Agnipath or “path of fire” in Hindi, will bring in men and women between the ages of 17-and-a-half and 21 for a four-year tenure, with only a quarter retained for longer periods.

Previously, soldiers have been recruited by the army, navy and air force separately and typically enter service for up to 17 years for the lowest ranks.

The shorter tenure has caused concern among potential recruits.

“Where will we go after working for only four years?” one young man, surrounded by fellow protesters in Bihar's Jehanabad district, told Reuters partner ANI. “We will be homeless after four years of service. So we have jammed the roads.”

Bihar and neighbouring Uttar Pradesh saw protests over the recruitment process for railway jobs in January this year, underlining India's persistent unemployment problem.

Varun Gandhi, a BJP lawmaker from Uttar Pradesh, in a letter to India's defence minister Rajnath Singh on Thursday said that 75 per cent of those recruited under the scheme would become unemployed after four years of service.

“Every year, this number will increase,” Gandhi said, according to a copy of the letter posted by him on social media.