Japanese pop band SMAP. Tsuyoshi Kusanagi is on the far left. Photograph: PR

Japan's best-known pop band faces an uncertain future after one of its members was found cavorting naked in a public park in Tokyo in the early hours today.

Tsuyoshi Kusanagi, a member of the hugely successful band Smap, was arrested on suspicion of public indecency after police officers found him seated naked on the grass near the Roppongi entertainment district at 3am.

Residents had called police after spotting a man wandering around the park and yelling incoherently.

The 34-year-old singer, whose clean-cut image has made him one of the most popular members of the five-piece band, reportedly said: "What's wrong with being naked?" His clothes were found in a bundle beside him.

Reports said Kusanagi, who has also built a successful film and TV career, had been drinking with friends in Roppongi before the incident.

His arrest has dominated the TV news in Japan, while fans lined the streets outside the police station for a glimpse of the singer.

While his antics were tame by the standards of the rock world, Smap's squeaky-clean image means their backers are likely to take a dim view of Kusanagi's drunken cavorting, even if their millions of fans will probably forgive him.

Toyota immediately suspended the rental car commercials in which he appears and industry experts speculated the band was likely to lose more lucrative contracts.

Smap – an acronym for sports, music, assemble, people – have dominated the pop scene for almost 20 years, as the most visible act belonging to the secretive talent agency, Johnny and Associates.

"We apologise deeply for the trouble and worry caused to everyone, especially his fans," the agency said in a statement, adding that it would make a further announcement on Kusanagi's future career.

Smap's members are fixtures on Japanese TV programmes, appearing in everything from variety to drama programmes and even hosting their own celebrity cookery show. They have built a huge following in South Korea, thanks mainly to Kusanagi's fluency in the language.

TV footage showed a haggard-looking Kusanagi being transferred to a police station this afternoon. "He has such a clean image," one woman told the TBS television broadcaster. "He doesn't seem like that kind of person."

Kusanagi reportedly apologised for his actions. He admitted stripping off in the park but said he had no recollection of how he got there.

The episode is an embarrassment for the government, which had hired Kusanagi to promote Japan's switch from analogue to digital TV broadcasting over the next two years.

The Kyodo news agency quoted Kunio Hatoyama, the internal affairs and communications minister, as describing the singer as a "bastard".

"His actions are shameful for a person who is asking the public to shoulder a financial burden," he said.