Mr Nadda accused Mr Gandhi of speaking the language of "anti-India" George Soros.
BJP president JP Nadda led his party's attack on Rahul Gandhi on Friday over the Congress leader's recent comments made in the UK on the state of Indian democracy, charging him with becoming a "permanent part of the toolkit" working against India that seeks a weak government here so as to exploit it for its benefits.
In a statement, Mr Nadda accused Mr Gandhi of speaking the language of "anti-India" George Soros, a billionaire financier, and alleged that the Congress and the "so-called Left liberals" have become a part of the "deep state" conspiracy involving foreign forces against the country. "Anti-national" Congress leaders have been speaking the language of Pakistan, he added.
Mr Gandhi will have to apologise to the people of India for his "sin" of seeking the "interference" of foreign powers in the country's internal affairs, the Bharatiya Janata Party (BJP) president said.
He accused the former Congress chief of joining hands with foreign conspirators against India to encircle the country economically and strategically. Never before in independent India any leader has done what Gandhi has done on foreign soil, he said.
"This is a most serious matter," Mr Nadda said, adding that what the Congress leader has done has hurt every patriotic MP as well as the people of the country.
Those working against India want a weak government working under the compulsions of an alliance for their benefits, he said.
The BJP president said anti-India forces have always had problems with a strong India, its robust democracy and decisive government. By criticising the state of democracy in India and seeking the intervention of the United States and Europe on foreign soil, Mr Gandhi has attacked the country's sovereignty, he added.
"Rejected repeatedly by people, Rahul Gandhi has become a permanent part of the 'toolkit' working against India," Mr Nadda said.
He accused the former Congress chief of insulting India, its Parliament, its democratically-elected government and people in Britain, a country that ruled over India for long. What Mr Gandhi has done amounts to strengthening those working against India, the BJP president said.
The Congress has dismissed the BJP's accusations and asserted that Mr Gandhi will not apologise. He has sought permission to speak in Parliament to respond to the criticism levelled against him by the ruling party members in both houses. Parliamentary proceedings have so far been a washout over the row.
Mr Nadda said India has become the fifth-largest economy and is hosting the G20 summit under Prime Minister Narendra Modi's leadership, while Mr Gandhi has chosen to "insult" the country on foreign soil.
"There can be nothing more shameful than seeking the US' and Europe's interference in India and claiming that democracy no longer exists in the country," he said.
Noting that Britain ruled over India before it gained independence, the BJP president said it was unfortunate that Mr Gandhi sought foreign interference from that country.
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)