By Rupam Jain and Shivangi Acharya

NEW DELHI - Indian opposition parties united on Sunday to protest against the arrest of a prominent leader weeks before a national election, accusing Prime Minister Narendra Modi and his party of rigging the vote and harassing them with large tax demands.

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

Delhi Chief Minister Arvind Kejriwal, a staunch Modi critic, anti-corruption crusader and a high-profile leader of the 27-member "INDIA" opposition bloc, was arrested on March 21 for alleged graft over granting liquor licenses, less than a month before voting starts in a general election widely expected to solidify Modi's mandate with a rare third term.

Kejriwal's Aam Aadmi Party says the case is fabricated and politically motivated. Modi's government and his Bharatiya Janata Party (BJP) deny political interference and say law enforcement agencies are doing their job.

"If the BJP wins this match-fixing election and changes the constitution, it will light the country on fire," said Gandhi, whose party ruled India for more than two-thirds of the time since independence in 1947 but has struggled since Modi swept to power a decade ago.

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

Sharing the stage with Gandhi at the popular Ramlila Maidan gathering site were opposition leaders including regional party heads who have overcome differences over which party would contest which seats.

BJP spokesperson Sudhanshu Trivedi told a news conference that the opposition rally, attended by thousands, was meant to hide their acts of corruption.

Congress, besides struggling with detentions and raids by India's financial crime-fighting agency, says it faces "tax terrorism" from large tax demands by the government and the freezing of some of its bank accounts, all of which it calls attempts to financially cripple the party.

Critics say Modi and his party have weaponised investigative agencies and tax authorities to cull political opponents and reduce the chances of a fair election, an accusation the BJP denies.

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