Buoyed by the polls and brimming with confidence, the leader of Britain’s opposition Labour Party, Keir Starmer, declared on Tuesday that he was ready to assume the mantle of power. But first he had to shake the glitter off his suit jacket.

As Mr. Starmer took the stage at his party’s annual conference in Liverpool, he was interrupted by a protester who rushed up behind him and showered him with green and blue glitter. It took a full seven seconds for security guards to run onstage and tackle the man, who shouted, “Politics needs an update!”

Taken aback but not thrown off his stride, Mr. Starmer shed his jacket, rolled up his shirt sleeves, and tried to turn the spectacle into an object lesson about the transformed party he now led.

“Protest or power,” he said, referring to Labour’s lengthy stint in opposition. “That’s why we changed our party.”

After the police dragged the protester from the stage, Mr. Starmer vowed to rebuild a Britain that he said had been broken by 13 years of Conservative rule, a project that he suggested would take two terms — a decade — in government.

“What is broken can be repaired; what is ruined can be rebuilt; wounds do heal,” Mr. Starmer said to a cheering crowd. “Today we turn the page, answer the question ‘why Labour?’ with a plan” for what he grandly proclaimed a “decade of national renewal.”

He offered no new announcements, but outlined an ambitious commitment to change planning laws to help build 1.5 million new houses, add more police officers, and overhaul the ailing National Health Service. He also promised to end preferential tax treatment for residents of Britain with non-domiciled status — a longstanding Labour goal but one with particular relevance given that the wealthy wife of Prime Minister Rishi Sunak, Akshata Murty, claimed that status until last year.

For Mr. Starmer, 61, an intense, serious-minded but charisma-challenged former prosecutor, the conference was vivid evidence of how far he has been able to pull his party out of the wilderness since he took over as leader after Labour’s crushing defeat in the general election in 2019, its worst performance since 1935.

Once torn by divisions between its hard-left and centrist factions, Labour projected near-lockstep unity in Liverpool, with little of the backbiting or protests that used to interrupt its gatherings. Once contaminated by anti-Semitism in its far-left ranks under Mr. Starmer’s predecessor, Jeremy Corbyn, the party this week expressed robust support for Israel in the aftermath of the bloody incursions into Israeli territory by Hamas fighters.

“I utterly condemn the senseless murder of men, women and children — including British citizens — in cold blood by the terrorists of Hamas,” Mr. Starmer said, to a standing ovation.

Except for a handful of activists waving Palestinian flags outside the conference hall, there was little evidence of the rancorous battles over the Middle East that used to rage at these gatherings.

Since becoming leader in 2020, Mr. Starmer has taken a “zero tolerance” approach to anti-Semitism, including purging Mr. Corbyn, who now sits as an independent lawmaker and will not be allowed to run as a Labour candidate in the next election.

At a panel on Palestinian rights held on the sidelines, the mood was cautious and subdued. Chris Hoyle, the director of the Council for Arab-British Understanding, warned the audience he would not tolerate “shouting and screaming.”

Labour’s decisive victory in a Scottish parliamentary election on Friday contributed to a sense of hopefulness, even inevitability, among the party faithful.

In a speech on Monday, Rachel Reeves, who leads economic policy for the party, said she looked forward to addressing the next conference as Britain’s first female chancellor of the Exchequer. She won a video endorsement from the former governor of the Bank of England, Mark Carney.

“You cannot trust the Tories with our economy ever again,” Ms. Reeves declared, pointing to the markets chaos that followed Liz Truss’s brief stint in office as prime minister last year.

Yet some worried that Labour’s confidence risked edging into hubris. While the party has an 18 percentage point lead over the Conservatives in polls, analysts warned that this lead is fragile, having to do more with voter frustration with the Tories than excitement about Labour.

Mr. Starmer has not reached anything like the levels of popularity that Tony Blair, another Labour leader, achieved before his landslide election victory in 1997. According to one recent survey, only a quarter of those asked said they were very or fairly clear about what Mr. Starmer stands for.

That partly reflects his decision to abandon some of the pledges on which he ran for the party’s leadership in 2020, including the nationalization of public services like water, energy and rail companies.

Mr. Starmer has also struggled to project a clear image to British voters. Opponents in the Conservative Party like to refer to his full title, “Sir” Keir Starmer, which he received for his “services to law and criminal justice” in 2014, but which they use to cast the Labour leader as a member of the elite.

On Tuesday, he played up his working-class roots, talking about how he was raised by his father, a toolmaker and his mother, a nurse. And he recalled how his dreams were incubated in his family’s semidetached house in Surrey, not far from London.

“That pebble-dashed semi was everything to my family,” he said.

The lineup of Labour speakers — all singing from the same song sheet — underscored Mr. Starmer’s ironclad control over the party. It contrasted starkly with last week’s Conservative Party conference, where Ms. Truss and an ambitious home secretary, Suella Braverman, seemed to be competing openly with Mr. Sunak to be the future of the party.

Mr. Starmer has moved swiftly to take control of Labour’s power structures, junking most of the polices of Mr. Corbyn, the most left-wing leader of the party in decades.

With the party under his thumb, he has developed a warm relationship with Mr. Blair and brought his former aides back into the fold. One of them, Peter Mandelson, a member of the House of Lords, said Britain’s economic woes would present a huge challenge for Mr. Starmer, if he wins power.

“The country is in a state of pessimism bordering on depression at the state of politics, given everything we have gone through in recent years,” Mr. Mandelson said. “He’s got to give them hope mixed with realism.”

Mr. Starmer struck that balance, warning that a Labour victory in 2024 would require equaling the achievements of great Labour victories in 1945, when a Labour government rebuilt Britain after World War II; 1964, when it modernized Britain’s sclerotic economy; and 1997, when Mr. Blair set about fixing crumbling public services.

“Britain can, Britain must, and Britain will get its future back,” Mr. Starmer said, the glitter sparkling in his hair. He received a rapturous ovation from the activists of his rejuvenated party.