Labour deputy leader Tom Watson has sought to calm fears among the party's MPs that they are being targeted by hard left supporters of Jeremy Corbyn.
Mr Watson dismissed the left wing pressure group Momentum as a "rabble" which was not as influential as it liked to claim.
At the same time he issued a warning to Mr Corbyn's critics in the Labour ranks that some of them had gone "too far" in their public denunciations of the leader.
His comments came as the party celebrated a better-than-expected result in the Oldham West and Royton by-election – comfortably holding the seat with an increased share of the vote.
It came as some respite for the leadership at the end of a tumultuous week in which deep divisions within the party were blown wide open in the Commons debate on air strikes on Syria.
Following Wednesday night's vote, there were complaints by MPs of orchestrated email and social media campaigns to pressure them into opposing military action, even though they had been given a free vote.
Former shadow cabinet member Tristram Hunt complained of the "reprehensible" intimidation of female MPs while backbencher Neil Coyle - who referred an alleged death threat to the police – urged Mr Corbyn to distance himself from Momentum.
Momentum, however, strongly denied plotting to de-select MPs who backed air strikes while Mr Watson said the group's influence had been overstated.
"They look like a bit of a rabble to me, but I don't think they are a problem for the Labour Party. I don't think they are effective. I think they are a bit of an irrelevance in the debate," he told the BBC Radio 4 Today programme.
Mr Watson also had some sharp words for Mr Corbyn's critics, saying some of their comments had been "deeply unhelpful" and calling on them to "swing together" in the wake of the Oldham West victory.
"If this was a referendum on Jeremy Corbyn, then he has won. It was a decisive victory with our share of the vote going up," he said.
"I hope our MPs look at this result. What's happened since Jeremy became leader and I became deputy leader is we have focused on issues that affect the working people of Britain. I think people responded to that at the ballot box."
Mr Corbyn travelled to Oldham to celebrate the result with the victorious Labour candidate – local council leader Jim McMahon – who held the seat with a majority of more than 10,000 over Ukip.
The Labour leader told cheering supporters: "It shows just how strong, how deep-rooted and how broad our party, the Labour Party, is for the whole of Britain."
However Ukip leader Nigel Farage said he would be lodging a formal complaint over alleged abuses of postal voting system, saying the result raised questions about the conduct of elections in constituencies with large ethnic minority communities.
"They can't speak English, they have never heard of Ukip or the Conservative Party, they haven't even heard of Jeremy Corbyn," he said.
"It means effectively that in some of these seats where people don't speak English and they sign up to postal votes, effectively the electoral process is now dead."
His allegations were dismissed as "sour grapes" by Labour.
In the poll triggered by the death of former minister Michael Meacher, Mr McMahon polled 17,209 votes, with Ukip's John Bickley trailing in second on 6,487, a majority of 10,722.
Labour's share of the vote increased by more than seven points to 62.1% in a 2.27% swing from Ukip.