An all-star show to celebrate the city’s emergence after the hardships of the pandemic, even as the spread of the Delta variant has driven up cases again, was stopped halfway through.It was supposed to be a glorious celebration of the re-emergence of New York City after more than a year of pandemic hardship — a concert bringing thousands of vaccinated fans on Saturday evening to the Great Lawn of Central Park to hear an all-star lineup.And for the first couple of hours it was, with messages of New York’s resilience sandwiched between performances by the New York Philharmonic, Jennifer Hudson, Carlos Santana, LL Cool J, and Earth, Wind and Fire, among others.But shortly after 7:30 p.m., as Barry Manilow was performing “Can’t Smile Without You,” lightning brought the concert to a halt. “Please seek shelter for your safety,” an announcer intoned, stopping the music, as people began filing out of the park.The concert had begun with a ray of sunshine, breaking through the clouds just before it got underway at 5 p.m. Gayle King, a co-host of “CBS This Morning,” began the evening by thanking the essential workers who had pulled the city through the darkest days of the pandemic.“We were once the epicenter of this virus, and now we’ve moved to being the epicenter of the recovery,” she said. “We gather for a common purpose: to say, ‘Welcome back, New York City!’”She then introduced the New York Philharmonic, which kicked off the concert with the overture to Leonard Bernstein’s “Candide,” conducted by Marin Alsop, a Bernstein protégée. The orchestra then played a medley of New York-themed music, including bits of Gershwin’s “Rhapsody in Blue,” Billy Joel’s “New York State of Mind” and “Theme from ‘New York, New York,’” the anthem made famous by Frank Sinatra, among others.The concert, “We Love NYC: The Homecoming Concert,” which was broadcast live on CNN, was part of Mayor Bill de Blasio’s plans to celebrate the city’s comeback after the pain and suffering of the pandemic.When the concert was announced by Mr. de Blasio in June, plunging coronavirus case numbers and rising vaccination figures had filled the city with hope.But circumstances have shifted considerably over the past two months. The spread of the highly contagious Delta variant has led some city businesses to postpone the return to their offices, prompted the city to institute vaccine mandates for indoor dining and entertainment and threatened to destabilize the wider concert business.On June 7, the day the concert was announced, the city was averaging 242 cases a day; the daily average is now more than 2,000 cases a day.With the Philharmonic still onstage, the concert continued with Andrea Bocelli, the star Italian tenor, singing “O Sole Mio,” and Jennifer Hudson, the star of the new Aretha Franklin biopic “Respect,” singing Puccini’s “Nessun Dorma” — a beloved aria that became associated with Franklin after she sang it at the Grammy Awards in 1998.As the crowd streamed in, the idea of New York’s return — whether a two-fisted vanquishing of a viral enemy or a premature declaration of victory — was on seemingly everyone’s mind.“This is our reopening — this is our invitation to get back to real life,” said Dean Dunagan, 52, of the Lower East Side, who had come to see Mr. Springsteen and had been waiting outside the park for four and a half hours before the gates were opened.“New York has been punched in the face every other decade, or whatever,” Mr. Dunagan said, “and we get right back up.”Just a few feet from him was Alexandra Gudaitis, a 24-year-old Paul Simon fan from the Upper West Side. “I’m scared this is going to be a mass spreader event, with the Delta,” she said.Still, she was one of the first fans through the door and rushed to the very front of the general-admission section with a few friends. They wore masks, and Ms. Gudaitis said they had chosen their spot because it seemed to have better access to fresh air.Some of the acts had only tenuous connections to New York. But the rap pioneer LL Cool J led a New York-centric ode to old-school hip-hop with Busta Rhymes, A Boogie Wit da Hoodie, French Montana, Melle Mel and Rev. Run of Run-DMC.Amid concerns about the spread of the Delta variant, the show required everyone 12 years old and older to show proof that they had had at least one dose of a vaccine. Gabriela Bhaskar/The New York TimesThe homecoming show required everyone 12 years old and up to show proof that they had had at least one dose of a vaccine; children younger than that, who are still ineligible for the vaccines, were required to wear masks.“When it comes to the concerts, they are outdoors — they are for vaccinated folks only,” the mayor had said on Wednesday. “We are definitely encouraging mask use. But I really want to emphasize the whole key here is vaccination.”The Central Park show came after the city had hosted a week of free hip-hop shows, with local heroes including Raekwon and Ghostface Killah in Staten Island, and KRS-One, Kool Moe Dee and Slick Rick in the Bronx. Tickets were required to attend the concert on the Great Lawn — most were free, but V.I.P. packages cost up to $5,000 — and the show was broadcast on television by CNN and on satellite radio by SiriusXM.The concert was programmed by Clive Davis, the 89-year-old music eminence, who, in an interview this week, stressed the role that music could play in shaping society.“It’s vital and important that New York be back,” he said.From the stage on Saturday night, Mr. Davis, a Brooklyn native, made a plea to the audience: “Tonight, I only ask one thing: When you’re having a great time, cheer loud — loud enough so they can hear you all the way in Brooklyn’s Crown Heights.”The concert ended before many of the headliners, including Bruce Springsteen and Paul Simon, got to perform. Mr. Davis said in the interview that after Mr. de Blasio asked him in May to put together the show, his first call had been to Mr. Springsteen.“I picked up the phone and told him we were going to celebrate New York City,” Mr. Davis recalled. “He said he would show up and wanted to do a duet.” That duet was to have been with Patti Smith, on “Because the Night,” a 1978 song they wrote together.The abbreviated concert came at an uncertain moment for the music industry. While some high-profile artists, including Garth Brooks, BTS and Nine Inch Nails, have canceled tour dates recently, the show is largely going on in the live-music business — but it hasn’t been easy. Concert protocols, in New York and elsewhere, have been in flux for months, as the federal authorities, local governments and businesses have adjusted to the changing realities of the virus.Broadway is requiring masks and proof of vaccinations as its theaters reopen, and Los Angeles County recently announced that it would require masks at large outdoor events such as baseball games at Dodger Stadium and concerts at the Hollywood Bowl.Mr. de Blasio has defended going ahead with the concert, noting that it was being held outdoors and for vaccinated people, even as some other events have been canceled. This year’s West Indian American Day parade in Brooklyn, for example, planned for Labor Day Weekend, has been canceled.The eyes of the concert industry have been on Chicago, where the Lollapalooza festival drew 400,000 over four days in late July and early August, amid concerns that it could turn into a “superspreader” event. The festival, which was held outdoors, required that attendees show proof of vaccination or a negative test. Last week, the city said that 203 people attending the show had tested positive afterward and that no hospitalizations or deaths had been reported. More