Rio’s civil police force said that together with the ministry of justice it “prevented a bomb attack that would have occurred at Lady Gaga’s concert in Copacabana” on Saturday night.

Officials said the superstar’s huge free gig on the beach, her first concert in Brazil since 2012, drew up to two million people.

Writing on X, Rio police said it had arrested an adult who was “responsible for the plot” as well as a teenager in the operation codenamed “Fake Monster” — a reference to the US pop diva’s pet name for her fans, “Little Monsters.”

It said they had recruited people online to “carry out attacks using improvised explosives” and Molotov cocktails as “a collective challenge” with the aim of “gaining notoriety on social media.”

The police added that the group behind the alleged plot “spread hate speech” and promoted radicalization and self-harm online.

Police carried out raids across Rio de Janeiro state as well as in the states of Sao Paulo, Rio Grande do Sul, and Mato Grosso.

“The police acted silently and without creating panic… We acted surgically to prevent the crime from happening,” Felipe Curi from Rio De Janeiro’s Civil Police said in a statement.

‘Scared sick’

Lady Gaga’s concert was the second million-strong show on Copacabana beach in a year after a huge concert last year by Madonna.

Security was tight, with some 5,000 officers, drones and both surveillance and facial recognition cameras deployed.

The 39-year-old star’s fans reacted with shock to the news of the foiled plot.

“THANK GOD. This would have had an impact on over 2 million people,” posted the LadyGagaFansTogether Instagram account, which has 307,000 followers.

Writing on the same account one fan said he was “scared sick about her (Gaga) being safe at all times.”

Lady Gaga on Sunday thanked her fans in Brazil on social media for “the biggest crowd for any woman in history.”

“Your heart shines so bright, your culture is so vibrant and special, I hope you know how grateful I am to have shared this historical moment with you,” she posted on X, formerly Twitter.

The gig was part of a strategy by Rio to hold mega-concerts in May in an effort to boost tourism during what is considered the low season.

All week, Gaga fans had flocked to Rio, a city famous for its annual Carnival that attracts hundreds of thousands of tourists from around the world.

Mayor Eduardo Paes has also hinted that he plans to bring in Irish rock band U2, although no date has been given.

Lady Gaga had kicked off her show perched more than two meters (6.5 feet) above the stage in a massive scarlet hoop gown which opened to reveal a vertical cage from which her dancers exited to a rendition of her 2011 song “Bloody Mary.”

“Brazil, I missed you so much,” screamed the pop star, who performed several tracks from her latest album, “Mayhem,” as well as favorites from throughout her career.