The one-kilometre-long march that culminated at the Gateway of India was also attended by the Congress, CPM, CPI, Trinamool Congress, National Conference, Samajwadi Party and other regional outfits as well. Rebel JD(U) leader Sharad Yadav and Gujarat Patidar leader Hardik Patel too was present.
Although the event is being seen as another attempt at building a united front to take on the BJP in parliament and test the ground for a coalition ahead of the 2019 elections, the participants said there was no political move behind the gathering.
"This is not about politics, this is not about opposition versus government, it's not about UPA vs NDA but it's about saving the Constitution," said Omar Abdullah.
Asked why they felt the Constitution was under threat, he replied, "Just look at the response to a movie. Even children travelling in school buses are not safe. If you are unable to protect children because someone is happy with a movie that is made, clearly not just Constitution, our very identity is under threat".
Trinamool Congress leader Dinesh Trivedi told NDTV, "The Constitution is under attack and that is why to reconsider where we are going it is our duty to salute the Constitution. At the moment, there is no political statement."
"There can be differences among parties but this is about the nation. Today everyone, even the right and the left, have come together," Patidar leader Hardik Patel told NDTV.
Mr Pawar said the parties would meet in New Delhi on January 29 to chart a way forward. "It would be a great disservice to the nation and the Constitution if we do not raise our voice," he added.
In another part of the city, the BJP launched a counter-assault led by Maharashtra Chief Minister Devendra Fadnavis. "They are not trying to save the Constitution but their own parties," he said.
He took on Congress president Rahul Gandhi who had recently asked the prime minister to answer why the gulf between the rich and the poor was widening. "Why should Modiji answer? You should. It's your party that ruled India for 60 years," Mr Fadnavis said.
There have been attempts at forging an opposition alliance after the Modi government came to power, the last successful one being the coming together of the JD(U), RJD and Congress in Bihar that led to the BJP's defeat in the 2015 assembly elections. But efforts to replicate the model in Uttar Pradesh failed. And last year, the Nitish Kumar-led JD(U) returned to the NDA, scuttling the chances of an alternative formation in the run up to 2019.