Anas Sarwar has insisted that the Scottish Conservatives have “nothing to offer” the people of Scotland.
The Scottish Labour leader criticised the party ahead of its conference in Aberdeen, which kicks off on Friday.
And he branded Boris Johnson as “utterly unfit” for office, with the Prime Minister set to speak at the event.
But, the Scottish Conservatives said it was “laughable” for Scottish Labour to claim that they are the only party capable of standing up to the SNP.
Sarwar launched the attack on his political opponents two weeks after his own party’s conference was held in Glasgow.
“The facts are clear – the Scottish Conservatives are the party of the past and have nothing to offer the people of Scotland,” said Sarwar.
“Only a week on from his screeching U-turn, Douglas Ross is playing second fiddle at conference to Boris Johnson – a man who has shown himself to be utterly unfit to be Prime Minister.
“From the cruel benefits cap to inaction in the face of the cost of living crisis, it is clear that the Tories are failing the most vulnerable in Scotland.”
Sarwar also warned of the threat to the union due to the actions of the Conservatives and the Prime Minister.
He continued: “The truth is that Boris Johnson and the Tories are a gift to the SNP.
“That’s why they are the biggest threat to the future of the United Kingdom.
“So I say directly to people who may have held their nose and voted Tory because you thought it was the best way to preserve the UK – Boris Johnson has let you down.”
The Scottish Labour leader also issued an appeal ahead of the Scottish council elections.
“Only one party is strong enough to stand up to Nicola Sturgeon and the SNP – Scottish Labour,” he added.
“This May 5 when you go to the ballot box choose a Scottish Labour candidate – and let’s build a better future for Scotland together.”
A Scottish Conservative spokesman hit back at Sarwar and insisted the party is “leading Scotland’s fightback” against the SNP.
“It is laughable that Scottish Labour are trying to claim they are the only party capable of standing up to the SNP. They’ve been in six council coalitions with the nationalists since 2017,” said the spokesman.
“They have gone backwards in every election and suffered their worst ever result at last year’s Holyrood election.
“The Scottish Conservatives secured 100,000 extra votes last year and are the real alternative to the SNP.
“We will show at our conference how we have more policies than ever before and are leading Scotland’s fightback against the SNP.”
Meanwhile, the SNP accused Johnson of failing to act over the cost of living crisis.
The party’s depute leader Keith Brown urged the Prime Minister to return to Downing Street following his visit to Scotland with a plan to tackle soaring bills.
“The people of Scotland have consistently rejected the Tories at the ballot box – yet have been lumped with a decade of crippling Tory austerity that we didn’t vote for,” he added.
“Boris Johnson makes the case for Scottish independence all by himself – so he shouldn’t be surprised about the result when that democratic mandate is delivered.”