Top leaders of India's opposition coalition and thousands of supporters rallied in the capital on 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 this month 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 under his Bharatiya Janata Party (BJP) banner as a foregone conclusion, partly due to the resonance of his assertive Hindu-nationalist politics with members of India's majority faith.

Several leaders of the two dozen political parties of the INDIA opposition alliance - the Indian National Developmental Inclusive Alliance - addressed the crowd.

"This is a fight to save India," Congress party leader Rahul Gandhi said. "If the BJP comes back to power, they will discard the constitution."

Kejriwal's wife Sunita read out what she said was a message from the arrested leader.

"I want us together to build a new India where there is justice for everyone," she read. (AFP)