The Labour Party must show "a new spirit of solidarity" and end internal splits and divisions, shadow chancellor John McDonnell has said.
Speaking to hundreds of Momentum activists, he said: "We're fed up with divisions and splits and arguments, we want to work together."
He added: "We've got to engender now in the Labour Party a new spirit of solidarity."
But he also spoke of ending "perennial leadership elections" and called for party-wide acknowledgement leader Jeremy Corbyn had been twice democratically elected.
Mr McDonnell also said Labour had "made mistakes", but added he would not be surprised if hostile media used that admission for its headlines.
And he claimed Mr Corbyn had been "set up" for a coup, following the Remain campaign's failure in last year's EU referendum.
Mr McDonnell was addressing the inaugural Momentum annual conference in Birmingham's trendy Digbeth area on Saturday, delivering a fresh call for party unity.
He started by remembering the Westminster terror attack, saying: "No-one will divide us, we are one race - the human race, and we stand together."
Outside, pamphleteers were handing out Labour Party Marxists-branded news-sheets with the headline "(Deputy Labour leader) Tom Watson inflicts further damage on Labour Party."
But inside, Mr McDonnell said it was time for an end to the splits and division inside the party.
Earlier, Mr Corbyn, in a pre-recorded message, told delegates: "Keep it up, keep it going and keep working together to build our movement to get Labour into government."
Mr McDonnell joked that, at the beginning of the Labour leadership campaign, nobody - not even Mr Corbyn - seriously expected him to win.
He said: "I certainly didn't and I chaired his campaign committee."
Mr McDonnell claimed the party had then been "on-track" after Mr Corbyn was first elected leader, but the EU referendum result had opened the door to a leadership "coup" attempt.
He added: "They set him up to blame for that referendum and use that as the excuse for the coup."
He also said: "We know we've made mistakes, no-one's perfect, but I tell you our ambitions are such to transform society and re-distribute wealth and power.
"It is no wonder that the oligarchs that own our media are coming at us.
"It is no wonder those that have the wealth and power are fighting so hard to retain it."
Mr McDonnell said the focus must now be on uniting behind key policy ideas like building new council houses, a fairer taxation system and protection for the NHS.
He added: "Those ideas will only come to fruition if we take the path of mobilisation again", adding it was only possible "in comradeship with everybody else".
The shadow chancellor said: "There are people in our party who we have different views with.
"We want to ensure that we work cooperatively with them.
"That we have that debate about those ideas, see if we can convince others about our ideas, and listen to theirs as well - listen to everybody.
"We want to build a party that is based on comradeship and solidarity.
"We're fed up with divisions and splits and arguments, we want to work together."
He joked: "If I can offer to have tea with Peter Mandelson, you can work with everybody else in this party as well."
Mr McDonnell said: "We've got to engender now in the Labour Party a new spirit of solidarity.
"I don't want leadership elections to be perennial affairs.
"We've elected a leader, we've elected him twice.
"We expect loyalty to that principle of democracy, not to the individual, but to the principle of democracy."