India's opposition parties have drawn half a million supporters to the eastern city of Kolkata for the largest show of force yet against Prime Minister Narendra Modi ahead of national elections due by May.

The massive "Unite India" rally held on Saturday brought together leaders of over 20 national and regional parties - many of whom have previously fought against each other during elections - to rail against Modi and his ruling Hindu nationalist Bharatiya Janata Party (BJP).

"The Modi government is past its expiry date," said Mamata Banerjee, chief minister of West Bengal state.

Her government, which organised the rally, had suggested four million would attend, but Kolkata Police chief Rajeev Kumar said 500,000 showed up in the eastern city.

Modi, who was inspecting military hardware in Gujarat state on Saturday, accused the opposition of acting in self-interest.

"The alliance is not against me, it's against India," he said, as quoted by the Hindustan Times newspaper.

Today, at historic #UnitedIndiaAtBrigade rally, lakhs of people from all over the country joined the galaxy of National Leaders of United India to take pledge to save the country, protect democracy, preserve federal structure and to build a stronger, progressive and united India pic.twitter.com/aQ7i5ZSjmZ — Mamata Banerjee (@MamataOfficial) January 19, 2019

'Remove BJP, save democracy'

Modi, whose BJP won a landslide victory in 2014 and now seeking another term, faces rising discontent over unemployment and economic inequality.

The BJP was defeated in three key state elections in December to the Congress, the main opposition party.

Congress leader Rahul Gandhi did not attend the rally but sent party representatives. His mother Sonia Gandhi, the party's former president, said it was "an important attempt to galvanise leaders to fight the Modi government".

Leaders of the Left parties were also missing from Saturday's line-up, reflecting tensions among opposition parties on who would be their prime ministerial candidate against Modi.

Speakers from India's myriad state, regional and caste-based parties criticised the prime minister and his BJP for what they alleged was divisive politics that pitted communities against each other.

The slogan "remove BJP, save democracy" was repeated by several leaders.

HD Deve Gowda, who was briefly prime minister in the 1990s, said regional parties were powerful but "cannot save India without uniting".

The opposition has not allied in a formal bloc, but some state outfits have joined forces as the election draws nearer.

They got together in Kolkata with no agenda but to stop BJP.



They talk of democracy in a state where the ruling party does not even allow local elections democratically.



130 crore Indians are watching their games with great amusement.



India will give them a befitting reply! — Narendra Modi (@narendramodi) January 19, 2019

Last week, two regional parties and former bitter rivals - the Samajwadi Party (SP) and the Bahujan Samaj Party (BSP) - announced an unlikely alliance to challenge Modi in Uttar Pradesh state, India's most populous and politically critical state.

With the formation of the SP-BSP alliance in a state that sends 80 members to lower house of Parliament., Modi's party faces a risk of losing elections, Banerjee said.

Arvind Kejriwal, chief minister of Delhi, said Modi had failed to fulfil his promises including job creation.

"Modi had promised to create 20 million jobs a year but after a faulty launch of national sales tax and demonetisation in 2016 more than 10 million jobs were lost," he said.

The BJP dismissed the prospects of an opposition alliance, questioning who would lead such a coalition.

Rising discontent

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India's unemployment rate hit 7.4 percent in December, highest in 15 months, while the number of people employed fell by nearly 11 million from a year ago, a report by the Mumbai-based Centre for Monitoring Indian Economy said this month.

Several leaders also spoke of the BJP's failure to tackle the crisis facing farmers.

Politicians at the rally said India's growth had slowed during Modi's term and their first priority was to defeat him, adding a replacement would be decided after the elections.

Modi is expected to detail a package worth more than one trillion rupees ($14bn) in his last budget on February 1, including benefits for farmers and other taxpayers.