Sonia Gandhi, the president of India's opposition Congress party, is being questioned by a government agency in connection with a corruption case.
She and her son Rahul Gandhi have been accused of misusing party funds to acquire valuable real estate through a convoluted financial deal.
The Gandhis deny all the allegations.
They accuse the governing Bharatiya Janata Party (BJP) of using federal law enforcement agencies for political vendetta.
On Thursday, Congress workers held protests as Mrs Gandhi left for the office of the Enforcement Directorate (ED), which investigates financial crimes.
She arrived at the ED office accompanied by Rahul Gandhi who was questioned in the same case in June. Mrs Gandhi was initially called for questioning at the same time, but her summons had to be deferred after she tested positive for Covid-19.
The 75-year-old leader was admitted to hospital and was discharged later in June.
This is the first time that Mrs Gandhi will be questioned by a federal law enforcement agency. BJP leaders have denied accusations by Congress party leaders that they are misusing federal institutions to settle political scores.
The case against the Gandhis has been brought by Subramanian Swamy, a politician from the governing Bharatiya Janata Party (BJP), who accuses them of misappropriating party funds to buy a firm that published the now-defunct National Herald newspaper.
What is the National Herald case about?
The National Herald newspaper was started in 1938 by Jawaharlal Nehru, India's first prime minister and Rahul Gandhi's great grandfather.
The newspaper was published by Associated Journals Limited (AJL) which was founded in 1937 with 5,000 other freedom fighters as its shareholders.
In 1947, when India won independence, Nehru resigned as chairman of the board of newspaper after taking over his role as PM.
But the Congress party continued to play a huge role in shaping the newspaper's ideology. Some of India's best-known journalists have worked at the daily, which continued to be funded by the Congress party.
The newspaper ceased operations in 2008 for financial reasons. In 2016, it was relaunched as a digital publication and is now widely seen as a Congress mouthpiece.
Mr Swamy has alleged that the Gandhis used Congress party funds and took over AJL to try to acquire real estate assets in several cities, including Delhi and Mumbai, which are worth more than 20bn rupees ($250 million; £208 million).
The party has denied this, describing it as "a strange case of alleged money laundering without any money".
In June, Rahul Gandhi was questioned for around 50 hours over five days by the ED in connection with the allegations. Leaders from his party held huge protests in solidarity with him.
They are expected to turn out in huge numbers on Thursday as well. But senior Congress leader and party spokesman Jairam Ramesh has tweeted that the Delhi police, which reports to the federal home ministry, was not allowing journalists to enter the Congress party's headquarters ahead of Mrs Gandhi's appearance.
Reports have said that senior leaders will accompany Mrs Gandhi as she leaves for the ED office. Opposition parties are expected to raise the issue in Parliament on Thursday.