The platform for opposition unity was the 'Sanjhi Virasat Bachao' (save our composite culture) meeting hosted by rebel JD(U) leader Sharad Yadav and attended by a host of leaders, including those from the Congress, CPI-M, CPI, SP, BSP, NCP, RJD, NC, JD-S and RLD.
Addressing the gathering, which included former prime minister Manmohan Singh as well as the CPI(M)'s Sitaram Yechury, CPI's D Raja and National Conference's Farooq Abdullah, Gandhi said they backed an India that stood for the truth.
He also stressed on the need for unity in the fight against the BJP and the RSS.
"If we all fight unitedly, I can tell you they will not be seen anywhere," he said.
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Sharad Yadav, who has parted ways with his old-time colleague, Bihar Chief Minister Nitish Kumar, amid speculation that the JD-U would split, echoed him.
If people united, "even a Hitler" cannot stand before them, said Yadav, who appeared to emerge as a rallying point for the opposition with leaders collectively endorsing his call for preserving India's composite culture.
Alleging that the RSS knew that its views would not help them win elections, he said it was trying to plant its people in every institution.
"Till the time RSS did not rule India, they never saluted the national flag... You can look at this nation in two ways. One says this country is mine, other says I belong to the country. That's the difference between RSS and us," he said.
Calling for a joint fight against "divisive and communal" forces, he said more Muslims chose to stay back in India than go to Pakistan after partition.
But this composite culture is now being tampered with to convert India into a "Hindu Rashtra", Yechury alleged.
Yechury said Yadav held the key to saving India's syncretic culture.
"We are with Sharad Yadav. If we have to save this composite culture, Sharad ji will hold the key to the movement to save it. He will have to complete the task," he said.
"We will fight against the policies destroying diversity," Yechury added.
Congress leader Ahmed Patel, who was recently elected to the Rajya Sabha from Gujarat, also backed Yadav.
"I urge Sharad ji to prepare an action plan... We will go to every nook and corner of the country and warn people that we will not be able to save the country, democracy, humanity if we do not save our composite culture, heritage.
Leader of Opposition in the Rajya Sabha Ghulam Nabi Azad said Yadav was the "real JD-U" and noted that he had declined a cabinet berth and chosen the path of secularism.
CPI's Raja dubbed the RSS an "unconstitutional" authority and alleged it was instructing the government in every matter.
"They are trying to destroy our composite culture by dividing the people. They want to win elections in any manner."
National Conference leader Abdullah also launched a scathing attack on the state of affairs in the country but did not name anybody.
"Andar koi chor baitha hua hai jo hamara beda garak kar raha hai (There is a thief sitting inside who is determined to spoil everything)," Abdullah added in an apparent broadside against the Centre.
Reacting to the meeting, BJP leader Ravi Shankar Prasad said it was "hypocrisy" to talk about composite heritage and remain silent on the killing of RSS workers in Kerala allegedly by CPI(M) goons.
"To this talk of composite heritage which is going on today, I have a question. The RSS workers in Kerala's Kannur who are being allegedly killed by CPI(M) workers, which heritage is that," he asked.