The "Save Democracy" rally, organised by an alliance of India's opposition parties, was the first major public demonstration against the arrest of Mr Modi's chief political rival Arvind Kejriwal earlier this month.

Mr Kejriwal, New Delhi's top elected official and a prominent anti-corruption campaigner, was arrested by the federal Enforcement Directorate, which is controlled by Mr Modi's government, on charges his party and ministers accepted one billion rupees (£9.5m) in bribes over the city's alcohol policy.

The Aam Aadmi Party, or Common Man's Party, says the case against Mr Kejriwal is fabricated and politically motivated. But Mr Modi's Bharatiya Janata Party (BJP) said the opposition rally was meant to hide their acts of corruption.

"Narendra Modi is trying match-fixing in this election," Congress party leader Rahul Gandhi told the rally in New Delhi as the crowd chanted: "Shame!"

Mr Gandhi's party has ruled India for more than two-thirds of the time since independence in 1947 but has struggled since Mr Modi swept to power a decade ago.

"If the BJP wins this match-fixing election and changes the constitution, it will light the country on fire," Mr Gandhi added.

"This is not an ordinary election. This election is to save the country, protect our constitution."

Mr Kejriwal's arrest is seen as a setback for the opposition bloc, who pose the main challenge to Mr Modi's party in the national elections to be held over six weeks from 19 April.

It is widely expected the election will hand Mr Modi a rare third term.

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The prime minister's critics have argued he has weaponised investigative agencies and tax authorities to cull political opponents and reduce the chances of a fair election, which the BJP denies.

"This fascism will not work in India," Mr Kejriwal's wife, Sunita, told the rally. "We will fight and we will win."