For the Israel Philharmonic Orchestra, marking its 75th anniversary, it was a jarring first, and in a city that has been one of the orchestra's most welcoming hosts: the repeated disruption of its concert at Royal Albert Hall in London on Thursday night by pro-Palestinian demonstrators, to the point that the BBC cut off its live broadcast and played recordings of the evening's program instead.

The protest was a first, too, for the BBC's Promenade Concerts, a popular rite of summer in Britain since 1895 that aims, promoters say, to bring classical music to the widest possible audience, performed by orchestras and ensembles from across the world.

According to the BBC, no other performance since the live "Proms" broadcasts began in the 1930s has been disrupted by protests. As for halting a broadcast altogether, a BBC spokesman said the only precedent anybody could remember was during German bombing raids on London in World War II.

If any other marker were needed, the concert was billed as a celebration of the 75th birthday earlier this year of Zubin Mehta, who has been musical director of the Israeli orchestra, known as the IPO, and before that the orchestra's musical adviser, for a total of more than 40 years.

On Friday, Mehta flew with the orchestra to Switzerland, on the next leg of a European tour, declining to comment on the protest. One music lover among the sellout audience of more than 5,000 in London described Mehta's demeanor as conductor, during the disruptions and the loud booing and shouts of "Out! Out! Out!" that others directed at the protesters, as one of stoic calm, with Mehta never looking at the audience and waiting with his baton at the ready until the uproar calmed down.

The Proms concerts are frequently joyous affairs, with an accent on informality. This year, they have featured an evening of musical comedy and a concert by an Australian group, the Spaghetti Western Orchestra, playing Ennio Morricone's scores for films like "A Fistful of Dollars" with instruments including asthma inhalers and cereal packets.

But the Israel Philharmonic performance was jolted by a group of about 30 protesters, whose interruptions appeared to have been carefully planned, said Rebecca Driver, a spokeswoman for BBC's Proms. The program, featuring works by Anton Webern, Max Bruch, Isaac Albeniz and Nikolai Rimsky-Korsakov, was disrupted four times in two hours, each time by a group seated apart from the last, forcing security officers to expel the protesters one group at a time.

The protesters included a core group of professional musicians, who gave what amounted to a counterconcert, breaking into vocal choruses in a bid to drown out the orchestra. A statement by the protesters said that they were members of "a new vocal ensemble" called Beethovians for Boycotting Israel, and described their behavior during the concert as a "debut performance."

The statement quoted one of the singer-protesters, Deborah Fink, whom it identified as a soprano, describing the group's first disruption as "intricately interwoven" with the Israel Philharmonic's first piece, Webern's Passacaglia.

"We thought we'd liven up the Webern a bit," Fink was quoted as saying. "The performance of Beethoven's Ninth Symphony at the previous night's Prom was so exciting that we decided to treat the audience to our own version of the 'Ode to Joy."' She then cited lines from the protesters' bitterly satirical version of the 18th-century poem by Friedrich Schiller that is sung to Beethoven's score:

Israel, end your occupation:

There's no peace on stolen land.

We'll sing out for liberation

Till you hear and understand.

Several concertgoers said that the audience appeared to be overwhelmingly hostile to the protesters, with boos, shouts and cries of "Shut up!" countering the protesters' choruses.

"It was just quite upsetting," said Tom Dixon, one of the concertgoers. "The Israelis had come just to play music, and they were being targeted for something I don't think they were involved in at all."

Another man in the hall, Jonathan Hoffman, said he had carried an Israeli flag to the concert and waved it during the protests. "I'm proud of the fact that I'm Jewish," he said. "I'm proud of this wonderful orchestra that consists of people who face terror every day."

Driver, the Proms spokeswoman, said the police officers who had been posted at the entrances to keep peace between pro-Israeli and pro-Palestinian protesters did not enter the hall and made no arrests.

She said that concerns about possible disruption inside the hall had prompted the organizers to mount an unusually thorough search of all the concertgoers, but that the checks had not discovered the patches of hand-lettered cloth that protesters inside the hall had used to spell out the words "Free Palestine" as they staged their demonstrations.

BBC Radio Three, the highbrow station that broadcasts the Proms, cut away quickly after the first of the protests broke out, apologizing to listeners for the interruption. It returned twice to the concert, resuming the broadcast, before finally abandoning it.

During the breaks, and after the final interruption, the station played recorded versions of the works on the orchestra's program. In the concert hall, Mehta led the orchestra through the full program, concluding to a huge ovation and an encore.

The possibility of disruption had already been well flagged. Earlier in the week, a letter signed by 23 professional musicians, including Fink, a music teacher who has been active in Jewish groups opposing the Israeli occupation of pre-1967 Palestinian territory, was published in a newspaper, The Independent, castigating the BBC for inviting the Israeli orchestra.

"Israel deliberately uses the arts to promote a misleading image of Israel," the signers said. "Through this campaign, officially called 'Brand Israel,' denials of human rights and violations of international law are hidden behind a cultural smokescreen. The IPO is perhaps Israel's prime asset in this campaign."

The conductor, Mehta, who was born and raised in India, kept his thoughts to himself after the concert, declining all requests for interviews. But perhaps anticipating trouble, he had given his views on the orchestra's political and social significance in an interview with the BBC before the performance.

"The IPO is a cultural ambassador for Israel, and it is not 'whitewashing Israel's crimes,"' he said, quoting a pro-Israel boycott campaign. "People who make music have to be politically aware; we have to know what's going on. We have to bring people together. In Israel, for Arab and Jewish audiences, and on quite a few occasions every year, at least for 2 1/2 hours, there is some sort of peace in the hall."