In an apparent swipe at the left-wing inner circle around Jeremy Corbyn, Ms Thornberry won loud applause from activists as she said Labour could not be reduced to “a protest movement”.
And she took a potshot at rival candidate Rebecca Long-Bailey over her claims of pizza-fuelled all-night working sessions, earning the riposte “Ooh, meow!” from the darling of the left.
Detailing her work in the shadow cabinet at a leadership hustings in Nottingham, Ms Thornberry said: “I know some people had to be up late eating pizza but I did all these difficult jobs at the same time in support of the leader in the way he needed to be supported.”
Her reference to Ms Long-Bailey’s disputed claim to have been forced to stay up all night to re-do the work of a colleague who quit the frontbench drew gasps from some in the audience.
Ms Long-Bailey responded: “Ooh, meow! We love each other really. I was working very hard, I will have everybody know. We did have to eat pizza.
“In terms of attacking colleagues, we shouldn’t attack each other publicly under any circumstances.”
The clash came as a poll for The Independent found Ms Thornberry trailing in last place among Labour voters in the race to succeed Mr Corbyn.
The BMG survey found 8 per cent of those who voted Labour in December want Thornberry as leader, against 9 per cent for Lisa Nandy, 12 per cent for Long-Bailey and 25 per cent for Keir Starmer. Among voters at large, Starmer led on 18 per cent, with the other three tied on 7 per cent, but most of those questioned had no favourite.
The shadow foreign secretary – who is struggling to secure a place in the final ballot alongside her three rivals – took aim at the community organising programme instituted by Corbyn and credited by party general secretary Jennie Formby for the party’s few successes on 12 December.
Ms Formby’s claim in an election post-mortem presented to the ruling National Executive Committee sparked resentment among some grassroots activists, who saw community organisers as an expensive bid to impose control from the party’s national HQ at Southside in Westminster.
There was loud applause as Ms Thornberry told today’s hustings: “I’ve met some community organisers who are bloody great but why is it that they are answerable to Southside and not to the community? I don’t understand that.”
She added: “We are the party created by the working people in order to be the party of power. We have to win power or we can’t do anything for local communities.
“Of course some people may want us to be a protest movement, but frankly the days of us being cuddly hopeless Lefties have got to be over. We can be cuddly, we can be Lefties, but we have to be in power.”
Ms Nandy also took aim at the community organising unit, telling the meeting: “Community organising as a model as the Labour Party has adopted it has completely and utterly failed.
“It has been very, very disrespectful to people campaigning out on the ground, and disrespectful to the people we represent.
“You don’t go into areas and tell people what they need and think and feel. We should never have become that party. I don’t ever want to hear again that people in Southside are sending people into areas to determine how resources are spent who candidate are and what campaigns are run.
“Labour was built as a party by, not just for, working class people in this country, and that is a sentiment we need to recapture.”
Ms Long-Bailey defended the work of community organisers as “critical”, telling the meeting: “Community building is key and we are only going to win a general election by rebuilding trust and making sure our communities know that the Labour Party is the community. Our councillors already do a great job in this, they don’t need to be told how to organise, they just need a bit of help and a bit of resource.”
Ms Thornberry defended her decision to stay on the frontbench under Mr Corbyn’s leadership, despite admitting it was “quite tough”.
“I remained in the shadow cabinet because my view was that the party belongs to the membership and the membership had chosen Jeremy as that leader and it was my duty to make him the best leader he possibly could be, and frankly it was quite tough,” she said.
“At one stage I was foreign secretary and defence secretary at the same time, at another stage I was foreign secretary and Brexit secretary at the same time. I know some people have to be up late eating pizza, but I did all these difficult jobs at the same time.”
Sir Keir was unable to attend the hustings, as his mother-in-law is seriously ill in hospital.
– BMG questioned 1,503 British adults between 4 and 7 February.