Jeremy Corbyn has hit out at Labour MPs who quit the party to sit alongside former Tories in the new Independent Group.
Speaking at the same rally, shadow foreign secretary Emily Thornberry said she would "rather die" than join any other party.
Mr Corbyn said MPs like Chris Leslie, who fought the last election on a Labour manifesto opposed to Conservative austerity, were now working with ex-Tories Anna Soubry, who supported the cuts.
Speaking at a rally in Ms Soubry’s Broxtowe constituency, he said that while he was “sad” some MPs had left, he had no intention of changing the policies which delivered the biggest increase in the Labour vote since 1945.
“I’m obviously very sad at some of the things that have happened and very sad at some of the things that have been said,” he said.
“Walking away from our movement achieves nothing.
"Not understanding where we have come from is a bad mistake.
“So, when the media talk about the bravery of those who walk away, Anna Soubry voted for austerity and said it was a good thing.
“Almost immediately after leaving Chris Leslie tells us that we should not be ending university fees, we should not be increasing the tax for the richest in the country, we should be cutting corporation tax and increasing the burden on others.
“I tell you what, the Labour Party believes in equality and justice.
"That is what was the centre of our manifesto and that is what will be at the centre of the next manifesto whenever that election comes.”
Also addressing the rally, Ms Thornberry told those gathered that they all had "a great deal in common and the biggest thing we have in common is the fact that we are Labour.
"We are Labour to the core and Labour to the tips of our fingerprints - and we would rather die than join any other party and we would never think of joining the other eight people who have decided to abandon Labour and cuddle up to the Tories.
"We would never do that."
The pair's speeches came at the end of a week which saw nine MPs quit the Labour Party – eight of them to join The Independent Group (TIG) – amid anger over the party’s position on Europe and anti-Semitism within its ranks.
Despite complaints by Jewish MP Luciana Berger and others that the leadership has consistently failed to deal with the issue within the party, Mr Corbyn insisted anti-Semitism was “unacceptable” in the party.
“When people are racist to each other, then we oppose it in any way whatsoever,” he said.
“If anyone is racist towards anyone else in our party – wrong. Out of court, out of order, totally and absolutely unacceptable.
“Anti-Semitism is unacceptable in any form and in any way whatsoever, and anywhere in our society.”