Indian National Congress party leader Rahul Gandhi (centre) shows his inked finger after casting his ballot to vote, at a polling station in New Delhi on Saturday. AFP

Key Indian opposition leader Rahul Gandhi voted Saturday as the country's six-week election resumed, with Prime Minister Narendra Modi's rivals accusing his government of unjustly targeting them in criminal probes.

Modi, 73, remains roundly popular after a decade in office and his ruling Bharatiya Janata Party (BJP) is widely expected to win a third term next month after a poll hit by recurrent early summer heatwaves.

His prospects have been further bolstered by several criminal investigations into his opponents, sparking concerns from UN rights chief Volker Turk and rights groups over the poll's fairness.

Gandhi, the most prominent leader of India's opposition Congress party, cast his ballot at a polling station in New Delhi, where temperatures were forecast to reach 43 degrees Celsius (109 degrees Fahrenheit).

A son, grandson and great-grandson of former prime ministers, Gandhi paused after voting to take a selfie with his mother Sonia but did not speak to crowds of reporters.

The scion of a dynasty that dominated Indian politics for decades, he was convicted of criminal libel last year after a complaint by a member of Modi's party.

His two-year prison sentence saw him disqualified from parliament until the verdict was suspended by a higher court.

Delhi Chief Minister Arvind Kejriwal, 55, leader of the opposition Aam Aadmi party, who was detained in March in a long-running graft case, was due to vote later Saturday.

The Supreme Court bailed Kejriwal earlier this month and he returned to the campaign trail, urging Indians to vote against what he called a nascent "dictatorship".

"Modi has started a very dangerous mission," he said soon after his release. "Modi will send all opposition leaders to jail."

Congress is spearheading an opposition alliance of more than two dozen parties competing jointly against Modi, including the Aam Aadmi party.

Kejriwal's organisation grew out of an anti-corruption movement a decade ago -- its name means Common Man's party -- and has been elected to office in the Delhi region and the state of Punjab, but has struggled to establish itself as a nationwide force.

Agence France-Presse