Top leaders of India's opposition coalition and thousands of supporters rallied in the capital Sunday, decrying "autocracy" in protest at the arrest of a senior colleague ahead of general elections.

Arvind Kejriwal, chief minister of Delhi and a key leader in an opposition alliance formed to compete against Prime Minister Narendra Modi, was detained earlier in March in connection with a long-running corruption probe.

Kejriwal's government is accused of receiving kickbacks while handing out liquor licences to private companies. Kejriwal, 55, denies the charges.

"The country is headed towards autocracy," Shiv Sena party leader Uddhav Thackeray, a former chief minister of Maharashtra state, told cheering crowds on Sunday. "This one-man government is taking the country to ruin."

Nearly a billion Indians will vote to elect a new government in six-week-long parliamentary elections starting on April 19, the largest democratic exercise in the world.

Many analysts see Modi's re-election as a foregone conclusion, partly due to the resonance of his assertive Hindu-nationalist politics with members of the country's majority faith.

Several leaders of the two dozen political parties of the INDIA opposition alliance -- the Indian National Developmental Inclusive Alliance -- are expected to address the rally.

India's main financial investigation agency, the Enforcement Directorate (ED), which arrested Kejriwal, has launched probes into at least four other state chief ministers or their family members.

While Modi enjoys high levels of support, critics accuse him of using law enforcement agencies to intimidate opposition leaders.

All the investigations involve political opponents of Modi's ruling Bharatiya Janata Party (BJP).

The flag-waving crowd also held up posters showing Kejriwal behind bars, with large numbers of police watching.