Indian investigators on Thursday summoned Sonia Gandhi, president of the opposition Congress party, for questioning on money laundering accusations that her allies have blamed on an abuse of government power.

The 75-year-old has been a driving influence in her once-dominant party since the 1991 assassination of her husband, former prime minister Rajiv Gandhi, and heir to a political dynasty that stretches back to the country's independence from Britain. 

Authorities are probing a decade-old complaint filed by a lawmaker from the ruling Bharatiya Janata Party (BJP), who accused her family of misusing Congress funds to buy a now-defunct newspaper business and personally acquire its highly valuable property portfolio.

The Gandhi family has denied the allegations and Congress has accused Prime Minister Narendra Modi's party of misusing its power. 

"Our nation's institutional agencies cannot be pawns in BJP's vendetta politics. The misuse of (the Enforcement Directorate) to attack opposition needs to stop," Congress tweeted Thursday.

Gandhi was initially summoned last month by the Enforcement Directorate, which probes financial crimes in India, but her questioning was delayed after she was hospitalised with a coronavirus infection.

Her son Rahul, who leads the party in parliament, was questioned for several days in June over the same allegations.

On Thursday, Sonia was accompanied by Rahul and her daughter Priyanka as she travelled to the Enforcement Directorate's office for questioning, arriving at around 1 pm local time, according to local media. 

Leaders of several opposition parties issued a joint statement Thursday condemning the investigation. 

Modi's government had "unleashed a relentless campaign against its opponents and critics through the mischievous misuse of investigative agencies," the letter read. 

"Prominent leaders of a number of political parties have been deliberately targeted and subjected to harassment in an unprecedented manner," its signatories wrote.