Scottish First Minister Alex Salmond has "lost the plot" after voters rejected independence, the leader of the campaign to keep the Union said.
Alistair Darling's comments to the Labour conference came after Mr Salmond claimed voters were "tricked" into voting "No" by Westminster leaders.
Mr Darling said his opponent must accept the outcome of the referendum.
Meanwhile, Labour reached out to supporters who voted for independence to come "home" to the party.
Scottish voters backed the country staying in the UK by 2,001,926 votes to 1,617,989 in Thursday's referendum.
Following the result, Mr Salmond, who is stepping down as first minister and SNP leader, said voters in Scotland were "tricked" by a late pledge from the leaders of the pro-Union parties to increase the Scottish Parliament's powers in the event of a "No" vote.
Labour, the Conservatives and the Liberal Democrats continue to disagree over how to deliver on their pre-referendum vow to devolve further powers to Scotland if it rejected independence.
But the UK government has denied that it is reneging on its commitment.
Analysis: Andrew Black, BBC Scotland political reporter
The voters may have rejected the SNP's defining policy of independence, but the party clearly isn't done fighting yet.
As Labour makes its appeal to win back supporters who voted "Yes" last Thursday, the SNP is talking itself up as a force to be reckoned with, suggesting it's on track to win the 2016 Holyrood election, citing a membership surge and promising poll predictions.
It has also turned to rhetoric which has served the party well in previous elections, that the SNP can be trusted to speak up for Scotland.
When the Scottish Parliament reconvenes on Tuesday for the first time since the referendum "No" vote, out-going First Minister Alex Salmond will make a statement to MSPs, in which he'll no doubt address the issue of more powers coming to Holyrood.
Mr Salmond says his opponents tricked people into voting "No" on Thursday, with a last-minute promise of new devolved powers, and the Scottish government now wants to put itself in a strong position to argue for as many new powers from Westminster as possible.
The problem with all this, say Scotland's pro-Union parties, is that Mr Salmond needs to fully accept the referendum result, while No 10 has dismissed claims that it's reneging on more devolved powers.
Meanwhile, the SNP leadership contest is on the horizon following Mr Salmond's decision to stand down, with current deputy Nicola Sturgeon expected to become the clear frontrunner to succeed him when nominations are due to open on Wednesday.
The new leader will take the reins at the SNP conference in November, after which Mr Salmond will stand down as first minister.
MSPs vote on his successor in that job, although the SNP's parliamentary majority will ensure it goes to their new leader.
Mr Darling, a Labour MP, who led the Better Together campaign during the referendum, told the conference in Manchester: "Some people haven't entirely accepted this result .
"Apparently the first minister of Scotland, Alex Salmond, said today, well he'd lost the referendum but never mind, he might be able to seize power some other way."
Mr Darling added: "I say this to Alex Salmond - you lost the argument, you lost the referendum, you've lost office and now you've lost the plot."
Also speaking at the conference, shadow Scottish secretary Margaret Curran said her party had to be honest about the fact that Labour voters had backed independence.
Speaking as Scotland's pro-independence parties reported surges in membership, Ms Curran said: "I know many good decent people driven by values similar to ours who voted 'Yes'.
"If you voted 'Yes' because you wanted a fairer and better Scotland, then we can work together. If you voted 'Yes' because poverty and inequality cuts you to the core, then come with us.
"And if you voted 'Yes' because you want to build a movement to change our country, I say the Scottish Labour party is not your opponent but your home."
Meanwhile, Scottish Finance Secretary John Swinney, speaking on the BBC's Good Morning Scotland programme, accused the UK parties of "utterly unseemly behaviour".
He argued that warnings from Mr Darling to deliver devolution promises showed that "a lot of games are being played at Westminster".
He added: "There should be absolutely no backsliding whatsoever."
Mr Swinney's comments came as Prime Minister David Cameron hosted a summit of senior Conservative MPs at Chequers to discuss plans to limit the Commons voting rights of Scottish MPs.
The prime minister has said a pledge to give Scotland more powers should go hand in hand with changing the role of Scottish politicians at Westminster.
But he is under pressure from Conservative backbenchers angry at the way Mr Cameron, Labour leader Ed Miliband and Lib Dem leader Nick Clegg teamed up to offer more powers to Scotland.
But Labour has also accused Mr Cameron of "playing the English nationalist card".
In other developments, nominations to replace Mr Salmond as SNP leader are expected to open on Wednesday, with current deputy leader Nicola Sturgeon the clear frontrunner.
Party members would also need to elect a new deputy leader, with the results of both elections declared at the SNP conference in November.