The BJP has said the conviction has come from an independent judiciary.
Top opposition leader Rahul Gandhi, former chief of the Congress party, was disqualified from parliament on Friday, a day after his conviction in a defamation case for a remark seen as an insult to Prime Minister Narendra Modi.
The Lok Sabha Secretariat also declared his constituency in Kerala's Wayanad vacant. The Election Commission can now announce a special election for the seat. Mr Gandhi will get a month to vacate his government bungalow.
The Congress called it a "conspiracy" to silence the leader, who has been dogged in his attacks on PM Modi and has been the de facto head of the main opposition party for nearly a decade.
The 52-year-old was convicted by a court and sentenced to prison for two years in Gujarat on Thursday for a 2019 speech in which he linked PM Modi's last name with two fugitive businessmen, remarking how the "thieves" shared the same last name.
The court also granted him bail and suspended the sentence for 30 days to allow him to appeal to a higher court. But according to the law, any MP convicted of a crime and sentenced to at least two years in jail stands to be disqualified.
Mr Gandhi, who debuted in parliament in 2004 and comes from a family that has given India three Prime Ministers, marched across India this year to revive the political fortunes of the Congress and galvanise its supporters.
The once-dominant Congress controls less than a tenth of the elected seats in parliament's lower house and has been decimated by the BJP in two successive general elections, most recently in 2019 under Mr Gandhi's leadership.
"Rahul Gandhi's Lok Sabha membership has been terminated. He is constantly fighting for you and this country, from the streets to the Parliament, trying everything possible to save democracy. Despite every conspiracy, he will continue this fight at all costs and will take just action in this matter. The fight continues," Congress said in a tweet in Hindi.
The party also changed its social media display picture to a vignette of Rahul Gandhi with the words 'Daro Mat' (Don't Be Afraid).
Senior Congress leader Manish Tewari called the decision "erroneous". "Lok Sabha secretariat cannot disqualify an MP. The President has to do it in consultation with the Election Commission," he told NDTV. Rajasthan Chief Minister Ashok Gehlot called it an example of "dictatorship".
Shashi Tharoor, another senior Congress MP, said he was "stunned" by the move.
Several opposition leaders have expressed shock, and targetted the BJP over the decision.
The BJP has said the conviction has come from an independent judiciary, with party president JP Nadda accusing Mr Gandhi of insulting an Other Backward Class (OBC) community - a key vote base for the party.
Rahu Gandhi's team has said they will challenge the verdict in a higher court. If the order is not cancelled, Mr Gandhi will not be allowed to contest elections for the next eight years.
Several political observers recalled how Mr Gandhi could have avoided the disqualification if an order by the Congress government introduced in 2013 that gave convicted lawmakers three months to retain their seats was still in place.
Mr Gandhi had publicly criticised the ordnance, famously calling it "complete nonsense" and recommending it should be "torn apart", making the Manmohan Singh government roll it back.
Several senior lawmakers have been disqualified from legislatures in the past. Indira Gandhi, Rahul Gandhi's grandmother, was briefly forced out of the chamber by a court decision in 1977 while she was Prime Minister.
But opposition parties say legal action against opposition party figures and institutions critical of the Modi government has seen a sharp rise in recent years.