In a relentless attack, she repeatedly contrasted Labour honesty with the "dishonesty" of Alex Salmond and his government as they sought to win the independence referendum.
Addressing the Scottish Labour conference in Perth, Lamont said Labour values had delivered greater fairness throughout history, while nationalism - "a grievance looking for a cause" - had not.
Urging co-operation, not separation, she said: "Nationalists tell us to have the confidence to leave the United Kingdom. I say to Scotland, have the confidence to lead the United Kingdom."
With the polls narrowing, and support for independence greatest in working-class communities, it was a speech aimed squarely at Labour supporters considering voting Yes.
It sought to portray the SNP's commitment to social justice as skin-deep, and the Nationalists as instinctively Tory. The SNP called it "desperate".
Lamont said the SNP had promised to cut corporation tax by 3p more than the Tories after a Yes vote, despite one of Salmond's economic advisers, Nobel-winner Professor Joseph Stiglitz, warning it was "a gift to the corporations, increasing inequality in our society".
She said: "So this Nationalist government knows that its key tax policy will increase inequality, not tackle it, but that is still at the heart of their plans.
"That is all you need to know about the kind of Scotland the Nationalists want to create.
"The Nationalists are running the most dishonest, deceptive and disgraceful political campaign this country has ever seen. Decent nationalists must be hanging their heads in shame."
The comments were made barely an hour after former Labour first minister Henry McLeish said the party must "stop hating Salmond and the SNP".
Claiming SNP policies were cynically crafted to win votes, and had no underlying philosophy, Lamont mocked Salmond's recent focus on childcare, and his offer of a "transformational" increase in free nursery provision under independence.
"There is one thing which the First Minister has discovered this year. Women give birth to children. Then they look after them," she said to applause.
Labour later denied the remark was a veiled reference to Salmond and his wife being childless.
Lamont said the childcare policy was being driven by the SNP's need to fix Salmond's "problem with women" and was possibly "the most patronising and cynical con even this government has come up with".
She said: "Transformational? What would be really transformational would be if this government ever told the truth.
"The Nationalists will go down as conducting the worst campaign of mis-selling in history. The Nationalists ask for an alternative to the White Paper. We have one. It's called the truth."
She also accused Finance Secretary John Swinney of viewing victims of the bedroom tax as "a political opportunity to be exploited".
Lamont spent one-quarter of the 40-minute speech defending her Devolution Commission, which reported last week.
It recommended giving Holyrood control over 15p of each income tax band to make it accountable for an extra £2 billion a year, devolving housing benefit, and giving more powers to councils.
Labour said it would use the powers to restore the 50p top rate of tax for Scots earning over £150,000.
However, the Commission back- tracked on a number of tax proposals floated in an interim report last year, including full devolution of income tax.
The changes followed a series of meetings between Lamont and shadow chancellor Ed Balls and his advisers, the Sunday Herald has learned.
Balls's team stressed the need to avoid tax competition between Scotland and the UK, and warned that full income tax devolution could jeopardise the Barnett formula on public spending, which gives Scots an extra £1364 per head over the UK average.
A senior party source said: "Ed Balls was sceptical of the interim report. He understands economics, while Johann does not."
Lamont ended with a focus on Labour values - and another attack on the SNP.
"I will do everything in my power to restore honesty to politics and remove this detached, dishonest SNP government.
"We will deliver on the next phase of devolution. We will stand up for Scotland's interests. We will crusade against injustice wherever it exists.
"Scottish Labour: a party of ambition; a party of honesty; a party committed to meeting the long-term challenges."
A spokesman for Alex Salmond said: "Scottish Labour and Johann Lamont appear obsessed with the SNP Government, and are still in denial about being out of office.
"If they want to be honest, they have to publish the results of their Cuts Commission, and tell us what social measures brought in by the SNP and previous administrations that they will scrap."