Gandhi, 52, lost his seat in parliament on Friday, a day after a court in the western state of Gujarat convicted him and sentenced him to two years in jail for a defamatory comment he made during an election campaign rally in 2019.
Gandhi is out on bail and his lawyers are expected to challenge the verdict in a higher court this week.
"The decision to disqualify Gandhi marks a black day in our country's democracy," Congress party president Mallikarjun Kharge, dressed in a black outfit and a black turban, told reporters outside parliament.
Gandhi told reporters on Saturday he was disqualified from parliament for questioning Prime Minister Narendra Modi about his relationship with Gautam Adani, founder of the Adani Group business conglomerate.
"We will continue to question Modi and his government for manipulating state-run assets to favour one company," Kharge said.
Gandhi and other opposition leaders say the prime minister's longstanding ties with Gautam Adani helped the business group secure investments from state-run firms and win big-ticket contracts such as the management of six airports.
Both the government and the Adani group have denied the accusations.
The Adani group is trying to rebuild investor confidence after United States short-seller Hindenburg Research accused it of stock manipulation and improper use of tax havens - charges the company has denied.
Hindenburg's January 24 report eroded more than $US100 billion ($A150 billion) in the value of the company's shares.
Opposition MPs have been vocal about their demand for a joint parliamentary committee inquiry into Adani's finances, forcing the Speaker to repeatedly adjourn proceedings of parliament.
A senior federal minister said Congress MPs were insulting parliament by not allowing it to function.
"They are determined to humiliate parliament, which is the temple of democracy," trade minister Piyush Goyal said on Monday.Â
