The New York City police have arrested 20 people for trying to enter an abandoned subway station housing the formerly secret guerrilla exhibition of underground street art that was revealed to the public this month.

The clandestine gallery has attracted urban explorers eager to catch a glimpse of dozens of provocative, large-scale installations created by more than 100 street artists who sneaked into the station over the course of a year.

Several of these spelunkers, however, have encountered something else: a team of police officers, some in plainclothes, assigned by the city to monitor the site. Most of those arrested were charged with trespassing and a few were caught carrying spray cans and other graffiti paraphernalia, the authorities said.

While the police are taking a hard line on keeping people away  “This is not an art gallery; this is completely illegal,” one officer said  the paintings in what the artists called the Underbelly Project are likely to live on. Subway officials said they had no plans to paint over the artwork, even if they sincerely hoped nobody ever got to see it again.