Labour faced a fresh internal row after an election to a key policy position was postponed amid allegations of "bullying".
The vote to elect a new chairman for the National Policy Forum (NPF) was put off after it was ruled that insufficient time had been allowed for the process.
Labour sources strongly denied the claims of bullying and insisted that the delay in the election was necessary to comply with party rules.
But some at the meeting angrily hit out at the way the decision was handled.
Labour MP Luciana Berger said she was "ashamed" to witness the "disgraceful treatment" of acting NPF chairwoman Katrina Murray at the event.
Fellow Labour MP Lucy Powell said the move to delay the vote "smacks of old-school control freakery".
Witnesses hit out at the behaviour of National Executive Committee (NEC) chairman Andy Kerr.
A source at the behind-closed-doors event in Leeds told the Press Association: "This morning symbolised the old-school male union bullying that is determined to keep Jeremy Corbyn's people in control no matter how bad it looks to the outside world."
There was speculation that veteran activist Ann Black had been on course to defeat Andi Fox in the vote.
Footage apparently shot inside the event showed Ms Powell debating with NEC chairman Mr Kerr, who insisted that the vote should not go ahead.
A source said: "Women delegates were coming up to Lucy Powell to thank her for trying to stand up to Andy Kerr, but in the end the burly ex-union guy took the female chair of the policy forum into a room and kept her there until he got his way.
"This hideous spectacle has underlined that the new Momentum tendency may have the muscle to dominate but they risk destroying any hope we have of convincing the public we are a credible alternative to the Tories."
But another source said: "That's absurd. The NEC is responsible for the rules and was merely making sure they were followed."
The Corbyn-supporting Momentum group denied it had pushed for the delay or endorsed any candidate.
It is understood the decision to delay the election was taken by the NEC officers group.
The only Momentum-backed member of the NEC officers group was not present at the meeting that decided to cancel the NPF election and the majority of members on the group are trade unionists.
A Momentum source said: "Only one out of eight members on the NEC officers group is Momentum backed, yet this still ends up being a 'Momentum plot'.
"This motion was carried by the trade unions - who have a clear majority on the group - but obviously that doesn't fit the paranoid fantasies of a small minority in the party."
A Labour source said the delay was required for the vote to be valid.
"For elections to be valid, seven days' notice is required according to party rules," the source said.
"That's why the NEC ruled out an election at its first opportunity. NPF officers can't call an election as it would have breached rules.
"The NPF can't overrule the NEC."
The NPF was already a source of discontent for some in the Labour ranks because of the lack of a specific policy commission looking at Brexit.
Some 17,000 emails have been sent to the party calling for the issue to be given its own commission with a full consultation of members' views.
Former minister Ben Bradshaw, a supporter of the Open Britain campaign, said: "I hope the entire National Policy Forum will agree that the absence of such a commission is problematic and inconsistent with our party's commitment to listening and responding to the views of our members."