There is "zero chance" of a successful coup against Jeremy Corbyn, Labour's new deputy leader said - despite conceding significant policy differences with him on key issues.
Tom Watson said the left-winger was putting together a "broad based" shadow cabinet and urged moderate colleagues to respect the "huge mandate" provided by his convincing leadership contest win.
But he accepted there were crucial areas - such as the new leader's opposition to the renewal of the Trident nuclear deterrent and his approach to Nato and Europe - where compromise would have to be sought.
Prime Minister David Cameron - who last night telephoned his new opponent to congratulate him - issued a stark warning that Labour under him posed "a threat to our national security, our economic security and your family's security".
Mr Corbyn was declared Ed Miliband's successor on Saturday on the back of a surge of support from activists that saw him attract a massive 59.5% of votes - topping the ballot among party members as well as trade unionists and new supporters.
The immediate resignations of a string of senior figures who declared themselves unable to serve in his top team swiftly underlined the task he faced uniting the party behind his anti-war and anti-austerity platform.
Senior party veterans such as Peter Mandelson and David Blunkett were among those warning that the party risked becoming unelectable without action to temper the new leader's radical mandate.
Mr Watson - appearing on BBC1's Andrew Marr Show in place of the leader, who pulled out to concentrate on shaping his new team - said he understood the concerns of moderate colleagues about the seismic political shift.
"He wants to build a broad-based party, he wants a front bench that represents all the talents and all the views," he said.
"So I say to colleagues: watch this space, respect the mandate he has been given from our members, try to unify and let's try and get an exciting programme for 2020.
Asked about reports of behind-the-scenes moves to oust Mr Corbyn, he insisted: "There is zero chance of that happening; Labour Party members will not accept that."
In a signal of the potentially divisive battles to come, Mr Watson - seen as a crucial linchpin in securing party unity - accepted that he and many other MPs backed the renewal of Trident.
"There are different views. Nobody wants to live in a nuclear world but it's how you disarm," he said.
"I think the deterrent has kept the peace in the world for half a century and I hope we can have that debate in the party".
He claimed there was no "massive disagreement" on Nato membership - but conceded he didn't know "precisely what Jeremy's position" was and hoped to "convince him of the merits".
"What I have read in the papers is that he actually understands that Nato needs to exist, he respects the views of many party MPs and members on this but he wants to make sure it doesn't expand eastwards.
"These things have got to be worked out."
"I aim to convince him of the merits of Nato. It was created by one of Labour's greatest ever foreign secretaries, Ernest Bevin, because his generation came through the Second World War and wanted to keep the peace."
Mr Watson said he believed the party could present a united front in favour of European Union membership - despite Mr Corbyn failing during the contest to make clear whether he would campaign to stay in at the forthcoming referendum.
"I will be a 'yes to Europe' definitely and I hope we can convince those sceptics in the Labour Party that that is where Labour needs to be in the referendum.
"I hope we can have our own distinct campaign that talks about a social Europe," he added.
Lord Blunkett said that if the party was allowed to lurch to the left, superficial public support for Mr Corbyn's regime would evaporate "when it becomes clear that they are disengaged from, and in a parallel universe to, the bulk of the electorate".
The election of Mr Corbyn "on an emotional spasm" showed "large swathes of the membership of the Labour Party are not on the same page as those who turned out to vote in a General Election", the ex-cabinet minister warned.
Justice Secretary Michael Gove said Tories should not be "gleeful" but "face up to that reality" that Mr Corbyn could be PM.
"It is because of that that we have to lay out at this time the scale of the risk that would be posed to Britain.
"The people he calls friends in the international sphere are the terrorist organisations Hamas and Hezbollah," he told Marr.