Keir Starmer said the general election amounts to a choice about "whose side are you on" as he sought to convince voters Labour is ready to govern during his first major speech of the campaign on Monday.
The Labour leader promised to stand up for working people and restore trust in British politics, saying 14 years of Conservative "chaos" has led to a "crisis" in national culture.
Speaking in Tory-held Lancing, West Sussex, he said: "Elections are about more than individual changes and policies, but about values, temperament, character and a bigger question: whose side are you on?
"Who do you hold in your mind's eye when you are making decisions?
"Everything I have fought for has been shaped by my life, every change I have made to this party has been about a cause, the answer to that question, the only answer: the working people of this country delivering on their aspirations, earning their respect, serving their interests."
Mr Starmer acknowledged that despite Labour's commanding opinion poll lead, many voters were not fully persuaded about his party.
"I know there are countless people who haven't decided how they'll vote in this election. They're fed-up with the failure, chaos and division of the Tories, but they still have questions about us: has Labour changed enough? Do I trust them with my money, our borders, our security?
"My answer is 'yes, you can', because I have changed this party permanently."
Mr Starmer took over the leadership of Labour in 2020, a year after the party suffered its worst general election defeat since 1935.
Keir Starmer through the years - in pictures
In an attack on the Conservatives' record, he said: "For a long time now, working people have believed opportunity in Britain is stacked against them.
"But now we are at a dangerous new point, close to crossing a Rubicon of trust, not just in politics but in many of the institutions that are meant to serve and protect the British people.
"A moment where people no longer believe their values or interests carry the respect of those in power.
"When you put that alongside a government that over 14 years has left living standards in this country worse than when they found them, that has torched any semblance of standards in public life, Westminster parties that broke the rules they put in place to save lives and rules they expected you to follow but ignored themselves, then you get a crisis in nothing less than who we are as a nation."
A teenage 'Dad's Army'
Prime Minister Rishi Sunak at the weekend announced the Conservative Party's plans to bring back National Service if it wins power at the July 4 general election, in the first major policy announcement of the campaign.
He said 18 year olds would be given a choice of joining the military full-time for 12 months or spending one weekend a month for a year “volunteering” in the community.
Mr Sunak said the policy would help unite society in an “increasingly uncertain world” and give young people a “shared sense of purpose”.
Mr Starmer dismissed the compulsory conscription plan as a "teenage Dad's Army".
He said the policy would be "paid for by cancelling levelling-up funding and money from tax avoidance that we would use to invest in our NHS".
"All elections are a choice and this is a clear one: levelling-up and the NHS with Labour or more desperate chaos with the Tories," he added.
Lib Dems in the mix?
Meanwhile, Liberal Democrat leader Ed Davey said his party could become the third biggest in the Commons after the election, overtaking the Scottish National Party (SNP).
Speaking ahead of the launch of the Scottish Lib Dems' election campaign, Mr Davey said he thought there would be "more liberals than nationalists" in the new parliament.
Speaking to BBC Radio Scotland on Monday, he highlighted four seats north of the border his party are targeting, all of which are currently held by SNP MPs.
These include Inverness, Skye and West Ross-shire – a portion of which was held by former Lib Dem leader Charles Kennedy before he lost it to the retiring former SNP Westminster boss Ian Blackford – and Mid Dunbartonshire, the newly configured constituency lost by former Lib Dem chief Jo Swinson to the SNP's Amy Callaghan in 2019.
"I think we have a real chance, when we get to July 5 – the day after polling day – there will be more liberals than nationalists in the next parliament," he said.
"I think the Liberal Democrats can be the third party in UK politics again."
