Indian opposition parties united on Sunday to protest against the arrest of a prominent leader weeks before a national election, accusing the 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,” the leader of the opposition Congress party, Rahul Gandhi, told a rally in New Delhi, as the crowd chanted “shame”. Delhi’s chief minister, Arvind Kejriwal – a staunch critic of Modi, an anti-corruption crusader and a high-profile leader of the INDIA opposition alliance – was arrested on 21 March for alleged graft over granting liquor licences, 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 against him 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. Gandhi said: “If the BJP wins this match-fixing election and changes the constitution, it will light the country on fire. This is not an ordinary election. This election is to save the country, protect our constitution.“ Gandhi’s Congress party ruled India for more than two-thirds of the time since the country gained independence from Britain in 1947, but has struggled since Modi swept to power a decade ago. Sharing the stage with Gandhi during the rally at the popular Ramlila Maidan gathering site in New Delhi were opposition leaders including regional party heads who have overcome their differences regarding which party would contest which seats. Sunita Kejriwal, the wife of Kejriwal, told the rally: “This fascism will not work in India. We will fight and we will win.” Modi said his fight against corruption had rattled the opposition. He said this election was a fight between his party and its allies, who want to remove the corrupt, and an opposition that wants to protect the corrupt. “Big corrupt people are behind bars and even the supreme court is not giving them bail,” Modi said in a rally to launch his election campaign in the populous northern state of Uttar Pradesh on Sunday. Congress – besides struggling with detentions and raids by India’s financial crime-fighting agency – says it is battling “tax terrorism” amid large tax demands by the government and the freezing of some of its bank accounts, which it says are 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.