The contest, usually a good-natured and camp celebration of pop music and European diversity now in its 70th year, has been plunged into crisis over Gaza.
National broadcasters from five nations, including Spain, the Netherlands and Ireland, are boycotting the 2026 event because Israel is taking part.
Viewers watching the semi-final on television heard distant chants of "Stop, stop the genocide" and "Free, free Palestine" as Noam Bettan began his performance of Michelle, which qualified for Saturday's final.
But many in the room, including a Reuters reporter, only heard cheers and whistles of support.
"I heard, first of all, in the beginning, a lot of booing, and it's fine. It's part of the thing. And a few seconds passed and I got a huge wave of unforgettable love and support," 28-year-old Bettan said in an interview.
"It really carried me onstage. And I looked for the Israeli flags ... and it gave me a lot of strength, and I really enjoyed the performance," he added.
The 2025 contest drew about 166 million viewers, according to its organiser, the European Broadcasting Union (EBU), more than the roughly 128 million who Nielsen estimates watched the Super Bowl.
Viewership is likely to be slightly lower in 2026 because the boycotting nations are not televising it.
The EBU and Austrian national broadcaster ORF, the 2026 host, issued a statement on what happened at the semi-final.
"One audience member, close to a microphone, loudly expressed their views as the Israeli artist began his performance, and during the song, which was heard on the live broadcast," it said.
"They were later removed for continuing to disturb the audience. Three other people were also removed from the arena for disruptive behaviour," it added.
The boycotting national broadcasters, which also include Iceland's and Slovenia's, have said they are protesting against Israel's military offensive in Gaza, which was in retaliation for the Hamas-led attack of October 7, 2023.
At least 1200 people were killed in the October 7 attack, most of them civilians. Israel responded by launching an assault on the enclave that killed more than 72,000 Palestinians, mostly civilians, and left much of Gaza in ruins.
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