Sir Keir Starmer has told Labour to "get serious about winning" in his first conference speech as leader.
Sir Keir attacked Boris Johnson's "serial incompetence" and said he was "just not up to the job" of being PM.
But he said Labour had work to do win back voters' trust - and had deserved to lose the 2019 general election.
In an online broadcast, he set out his vision for the UK and urged voters to "take another look at Labour", adding: "We are under new leadership."
And he assured voters who deserted the party in droves in some of its traditional heartlands: "We love this country as you do."
The speech was delivered to a near-empty room, in Doncaster, South Yorkshire, because of social distancing restrictions, and was greeted with silence instead of the usual applause and ovations.
He criticised the government's handling of the coronavirus pandemic and said the crisis had revealed Boris Johnson as being "just not serious" and "not up to the job".
"It makes me angry that, just when the country needs leadership, we get serial incompetence," said the Labour leader.
But he said Labour had to be "brutally honest" with itself about why had it had lost four general elections in a row.
"When you lose an election in a democracy you deserve to. You don't look at the electorate and ask them: 'what were you thinking?'," he told the party.
He said Labour was "becoming a competent, credible Opposition but that's not enough".
And he vowed that "never again will Labour go into an election not being trusted on national security, with your job, with your community and with your money. That's what being under new leadership means."
Brutal to his party, brutal about the PM
Keir Starmer literally stood in front of a red wall in Doncaster, and his key message was to those who had abandoned Labour in 2019 - we hear you.
He has something of an innovative policy chief in Claire Ainsley - one who doesn't believe in setting out detailed policies.
So he was blatant - clever policy offers aren't enough to win trust.
Far more important are values. He emphasised the classic Labour values of "compassion" and "opportunity", but also stressed the importance of family and security - seen as a weak spot for Labour under Jeremy Corbyn.
His message to his own party was more brutal than anticipated - suggesting that it would be a "betrayal" to be anything other than relentless in seeking power.
But he was also brutal towards the prime minister - Starmer was a serious lawyer when Boris Johnson was writing flippant columns.
So far, Starmer's focus as Labour leader has been on competence.
Polling suggests this has been successful - but he has been accused of being managerial, even funereal, in his approach.
Today he succeeded in injecting passion into a speech in a near empty hall.
His "new leadership" is ambitious for change, but he was also "angry" that he wasn't in power to achieve it.
Labour's four-day online event replaced the party's traditional party conference due to be held in Liverpool, which was cancelled due to the Covid-19 pandemic.
In his speech, Sir Keir said the policies on which Labour will fight the next election in 2024 "won't sound like anything you've heard before".
"It will sound like the future arriving," he said of the party's next manifesto.
On Brexit, Sir Keir, who campaigned for a second referendum, declared that Labour would not be a party "that keeps banging on about Europe".
"Let me absolutely clear," he said, "the debate between Leave and Remain is over."
He called on the prime minister to secure a trade deal with the EU, and said he would be "failing Britain" if he failed to achieve one.
He also pledged to fight for the Union, saying: "We must make the case much more persuasively that we achieve more together than we do alone.
"To stop the Nationalists ripping our country apart by design, and to stop the Tories dismantling it by neglect."
Sir Keir replaced Jeremy Corbyn as leader in the contest that followed December's general election, where the party won fewer Commons seats than any election since 1935.
On the economy, the Labour leader repeated calls for "properly funded" public services and "huge investment" in skills to create work opportunities.
He also called for action to tackle climate change, and for the government to develop a new strategy to close gaps in education inequality.
And he pledged to work "hand-in-hand" with both the private sector and trade unions in a bid to create "high quality jobs".
Momentum, the left wing campaign group originally set up to support Jeremy Corbyn's leadership, said Sir Keir's speech was a "missed opportunity to show substance".
The group's co-chair, and West Midlands regional secretary of the Fire Brigades Union, Andrew Scattergood, said: "If Starmer wants to appeal to working class voters, his pitch should be based on solidarity with the working class and defending their interests, not just slogans and platitudes."
He accused the Labour leader of being silent on key issues, including Black Lives Matter and the end of the evictions ban in England - and suggested he was "rowing back" on promises made during his leadership campaign, such as taxing the "super rich".
But Nathan Yeowell, director of Progress, which was originally set up to campaign for Blairite values, tweeted: "It is so refreshing to have a Labour leader who wants to win."
In a message to supporters after the Labour leader's speech, the Conservative Party said: "Sir Keir Starmer was a key member of Jeremy Corbyn's top team.
"Even now whilst he pretends to be showing 'new leadership' he can't even name a single one of Corbyn's polices he'd drop."