Keir Starmer condemned Boris Johnson as “trivial” and a “showman” as he brushed off hecklers to deliver a highly personal conference speech aimed at convincing voters Labour is ready for government.

The Labour leader received a string of standing ovations as he delivered his 90-minute speech to a packed hall in Brighton, closing a five-day conference in which his party’s divisions had been on stark display.

He repeatedly pressed home the message that Labour’s priority was now to win the next general election – and was applauded as he listed the achievements of the Blair and Brown governments.

Labour strategists believe the botched exit from Afghanistan and the chaos of this week’s fuel crisis have sparked fresh doubt in voters’ minds about Johnson’s competence.

Starmer accused the government of “ignoring the problem, blaming someone else, then coming up with a half-baked solution”. He ridiculed Johnson’s “levelling up” slogan, saying: “Level up? You can’t even fill up!”

“We have a fuel crisis, a pay crisis, a goods crisis and a cost of living crisis – all at the same time,” Starmer said, urging the prime minister to “either get a grip or get out of the way and let us clear up this mess”.

Starmer was introduced by Lady Lawrence, the mother of the murdered teenager Stephen Lawrence. He was joined in the audience by the parents of Jane Clough, who was killed by her former partner, and contrasted his legal career campaigning against injustice in situations such as theirs to Johnson’s past as a columnist and occasional TV guest.

In stark contrast to Angela Rayner’s controversial branding of Johnson as “scum”, Starmer sought to dismiss the prime minister as “a showman with nothing left to show” and “a trickster who has performed his one trick”.

“It’s easy to comfort yourself that your opponents are bad people. But I don’t think Boris Johnson is a bad man. I think he is a trivial man,” he said.

Starmer was repeatedly heckled by a small number of individuals, some of whom shouted “£15!” in reference to calls for a £15-an-hour minimum wage. He tackled them with a series of prepared putdowns, prompting applause from the majority of the audience in the hall.

Labour divisions were exposed afresh in Brighton after Starmer pushed through changes to the way the party elects its leaders and reselects sitting MPs at the weekend.

Grassroots members also passed a series of radical policy motions, including a demand for a “socialist green new deal” and a £15 minimum wage, which Starmer has said he would not support.

He did use the phrase “green new deal” in his speech, however, as part of a lengthy passage with echoes of Harold Wilson’s “white heat of technology” speech in which he highlighted the potential for transformative new technologies.

Starmer announced what he called a “national mission” to ensure every home in the country was better insulated and cheaper to heat within a decade. Labour calculates the £6bn-a-year plan would enable it to fund grants and low-cost loans to enable 19m homes to be insulated, helping consumers to cut their bills.

The speech was peppered with personal anecdotes about what Starmer called his “two rocks” – family and work – and he embraced high and low culture, from walking onstage to a track by the Brighton DJ Fatboy Slim – with whom Starmer had music lessons as a child – to quoting the poet WH Auden.

“You need not see what someone is doing to know if it is his vocation, you have only to watch his eyes. How beautiful it is, that eye-on-the-object look,” Starmer said, adding that he had seen that look in his toolmaker father.

He repeatedly portrayed Labour as on the side of working people, underlining the values of “work, care, equality and security”.

Starmer’s team believe the fluidity of public opinion means they have a chance of winning the next general election, though they acknowledge they face an uphill struggle against the Tories’ 80-seat majority.

With Labour keen to make gains in Scotland, where it was all but wiped out in the wake of the 2014 independence referendum and has struggled to recover, Starmer attacked Nicola Sturgeon’s record in office and promised Labour would offer a robust defence of the union. “We are more progressive together. We are more secure together. We are a bigger presence in the world together. We are greater as Britain than we would be apart.”

Without naming his predecessor, Starmer firmly distanced himself from the era of Jeremy Corbyn, promising never again to go to the electorate with “a manifesto that is not a serious plan for government” – prompting a heckler to shout: “It was your Brexit policy.”

Some of Starmer’s former shadow cabinet colleagues, including Jon Trickett and Diane Abbott, subsequently pointed out that Starmer had signed off on the manifesto for the 2019 general election, in which the party plunged to its worst defeat since 1935. Asked about this afterwards, Starmer’s spokesperson said: “There is a principle of collective responsibility.”

In a further striking contrast with the Corbyn years, Starmer stressed Labour’s patriotism and led the crowd in a standing ovation for Britain’s armed forces – though not all of those present joined in.

“Here in this conference hall we are patriots,” he said. “When we discuss the fine young men and women who represent all our nations we don’t boo. We get to our feet and cheer.”

Starmer’s team believe the perception that Labour was unpatriotic, or sympathetic to anti-western forces, was an important factor in the loss of scores of former heartlands seats in the last general election.

The Conservatives will gather for their own conference in Manchester this weekend, against the backdrop of petrol shortages, soaring energy bills and fears about the supply of key goods in the run-up to Christmas.

The longest speech? How Starmer compared
Starmer’s conference speech was the longest in recent memory, clocking in at about 90 minutes. His word count was just short of Ed Miliband’s 2014 speech: 8000 words compared with Starmer’s 7,300. A Labour spokesperson claimed Wednesday’s speech took longer to deliver because they had not anticipated the amount of clapping. Here are some of the other longest political speeches:

Ed Miliband, 2014

The former Labour leader gave one of the most memorable party conference speeches as he talked for over an hour without notes. He did miss out a long passage about the economy, for which he was heavily criticised.

Hugh Gaitskell, 1962

The then leader of the opposition gave a very long speech about his aversion to joining the common market. It had more than 10,000 words, but was delivered more quickly, in 83 minutes.

Fidel Castro, 1960

One of the longest ever speeches on the international stage was the Cuban leader’s speech to the United Nations on imperialism and colonialism. It ran to four hours and 29 minutes.

Henry Brougham, 1828

The prize for the longest continuous speech in UK parliament goes to this Whig MP, who spoke for more than six hours on law reform.