The British band Massive Attack’s position on Palestinians has never been in doubt. In recent years, they have called for a cease-fire in Gaza and often declared solidarity with Palestinians at concerts. In April, a member was arrested at a protest in London for supporting a pro-Palestinian group.

So it was no surprise that at a concert in Singapore last week, the band unfurled a Palestinian flag. But that led to punitive action by the local authorities.

In an Instagram post on Sunday, the group said its members were detained and questioned by the police after the show at The Star Theater on Wednesday. Some had their hotel rooms searched and their passports temporarily confiscated, the statement said, a sequence of events that left them “surprised and disappointed.”

The post added that large sections of the crowd had “organically” been leading chants of “Free Palestine” before the show, where group members Robert Del Naja and Grant Marshall later waved a Palestinian flag.

On Friday, the Singaporean police said in a joint statement with the country’s internet and content regulator that the display violated a law that criminalizes unauthorized displays of foreign flags, as well as another governing public gatherings. The former carries a penalty of up to 500 Singapore dollars, about $390, or six months in jail. Mr. Del Naja and Mr. Marshall were banned from entering the country in the future.

The authorities cast the musicians’ action as one that could “potentially harm the racial and religious harmony in Singapore.” They urged “the public, including foreigners, to refrain from importing foreign politics as this can undermine our social cohesion and the rule of law.”

But critics have long argued that Singapore’s racial harmony laws stifle freedom of speech, and Massive Attack is not the first musical act to run into trouble in Singapore.

In the 1970s, Led Zeppelin canceled a concert after being informed that they would be unable to enter Singapore unless they cut their long hair, which was at the time banned over concerns of hippie culture corrupting the country’s youth. Madonna was barred from performing in 1993 on moral and religious grounds, although she was allowed in to perform at a significantly toned-down concert in 2016. Last year, the Singaporean rapper Subhas Nair was jailed for six weeks for a YouTube performance that authorities said promoted racial hostility.

Massive Attack, pioneers of the trip-hop genre, have for decades been one of the music industry’s earliest and staunchest proponents of a Palestinian state, a position they have repeatedly hammered home throughout Israel’s war in Gaza, both onstage and on social media.

Singapore has long sought to strike a neutral tone in what officials have called the “emotive” issues of the Middle East, despite seeing Israel as a crucial defense partner that helped the country build up its military when it gained independence in 1965.

“We had to keep our defense cooperation low-profile to avoid riling up sensitivities,” Winston Choo, the former chief of the Singapore Armed Forces, recalled in an essay published in 2019.

In recent years, Singaporean authorities have forbidden public gatherings related to Israel’s war in Gaza, while also warning foreign travelers that wearing clothing related to the issue may result in being refused entry into the country.

Singapore does not recognize Palestine as a sovereign state. Last year, Singapore Foreign Minister Vivian Balakrishnan said that it supports a two-state solution when Palestinians have an “effective government that accepts Israel’s right to exist and categorically renounces terrorism.”

On Saturday, Massive Attack performed at the Pentaport Rock Festival in Incheon, South Korea, the next stop of their international tour after Singapore. Projected on the screen overlooking the stage was a large Palestinian flag.