Yogendra Yadav had parted ways with Arvind Kejriwal's AAP and formed Swaraj Abhiyan.
Civil society groups, trade unions and political parties - including the AAP and Swaraj India - on Monday joined hands against what they described as the onslaught of "right wing fascism".
The forum, calling itself a "democratic coalition to rescue democracy", has announced that it will take out a rally on October 5 in the national capital protesting issues such as the killing of journalist Gauri Lankesh and lynchings in the name of cow protection.
The coming together of Arvind Kejriwal-led AAP and Yogendra Yadav-led Swaraj Abhiyan assumes significance, considering their bitter parting of ways in the past, but this is not the first time that they are backing the same cause.
"Now is the time to respond. We earnestly appeal to all citizens who have shown the courage to say no to perpetrators of terror to build a broad, democratic coalition to rescue freedom and democracy from the onslaught of right wing fascism," a statement released by the forum said.
The October 5 rally will take place between Mandi House and Jantar Mantar at 1 pm, it said.
In the recent past, these parties unitedly backed the Narmada Bachao movement. AAP leader Dilip Pandey, Swaraj India's Yadav, Dalit rights activist Jignesh Mewani, CPI(ML) Red Star's K N Ramachandran, and social activist Harsh Mander made the announcement at a press conference here.
"We should stop being in denial that all this violence is happening by chance. It is taking place by design.
"Symbols of dissent are being silenced. AAP leader Ashish Khetan has received threats from Hindu extremist groups. The solution lies in struggle and we will back all such movements," Pandey said.