Millions of Indians went on a strike on Wednesday, unions said, as workers angry at the government's labour policies brought travel chaos across the country.

The widespread action was in opposition to what unions called the "anti-worker and anti-people policies" of right-wing Prime Minister Narendra Modi.

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They highlighted the recent privatisation of public firms and natural resources and demanded a rise in the minimum wage and pensions.

Farmers and students joined some of the protests called by 10 trade unions who claim 250 million members between them.

Students in many universities and colleges are boycotting classes in solidarity with attacks on students [Rupak De Chowdhuri/Reuters]

The nationwide strike comes amid a boycott of classes by students in many universities and colleges across India in solidarity with the students of Jawaharlal Nehru University (JNU), Jamia Millia Islamia (JMI) university and Aligarh Muslim University (AMU) who have come under attack from police, authorities and right-wing outfits.

Bus and train services and state-owned banks bore the brunt of the strike, staged in defiance of government warnings of "consequences" for anyone who took part.

Roads, train tracks blocked

Protesters blocked roads and railway tracks, while those at rallies also chanted slogans against the Hindu nationalist government's new nationality law that opponents say is anti-Muslim and has sparked widespread demonstrations.

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In West Bengal state, strikers targeted railways and key highways and burned tyres on a main road in the state capital of Kolkata. Protesters carried red flags at rallies in the city.

Police said there had been some "clashes" in Kolkata and other districts between supporters and opponents of the strike.

The eastern states of Bihar and Odisha, Maharashtra in the west, Haryana in the north and Kerala and Karnataka in the south were also hit by the strike.

Workers at state-run oil and coal firms joined the strike as they oppose government plans to privatise state-run companies. Bank employees are also participating in the strike against the proposed merger of public-sector banks.

'Severe economic slowdown'

CH Venkatachalam, general secretary of the All India Bank Employees' Association, said the proposed merger of 10 public sector banks into four banks would affect jobs and could hit the recovery of bad loans amounting to nearly $140bn.

Commuters make way on the Howrah Bridge during a nationwide general strike [Dibyangshu Sarkar/AFP]

"Modi government's policies have led to a severe economic slowdown and have created bad loans for banks," Venkatachalam told Reuters news agency by phone.

The government had warned strikers that they would face "consequences", including wage deduction and other disciplinary action if they joined the protest.

But the strike added to pressure on the Modi government already hit by widespread protests against the controversial nationality law passed on December 11. More than 25 people have been killed in protests. Growing unrest in universities has added to social tensions.

"The attitude of the government is that of contempt towards labour," said the Centre of Indian Trade Unions, one of the groups organising Wednesday's 24-hour strike.

Opposition parties have backed the strikers. Rahul Gandhi, former chief of the main opposition Congress party, said the government had "created catastrophic unemployment" in a Twitter message praising the action.

A government move to privatise big state-run firms as it struggles to get out of a severe economic slowdown has riled the opposition and unions.

Thousands of people have lost jobs in the manufacturing and construction sectors and debt-ridden companies have cut their investment plans.

Asia's third-largest economy is facing its worst slowdown in decades, and the government on Tuesday forecast 5 percent growth for the current financial year, the slowest in 11 years.