Bieber fever has hit an all-time high.

Teen pop phenom Justin Bieber, 17, is currently holed up in his hotel room in Liverpool, England -- with thousands of screaming fans surrounding the building outside.

"I cant even say hi to my fans outside because of the police," he tweeted Thursday. "Some bull."

Ironically, the Canadian-born singer is staying at the Beatles-themed Hard Days Night Hotel; the fab four were also hounded by overzealous fans in their heyday.

According to TMZ, police have warned the "Never Say Never" singer that his swarming fans could create a "possible riot situation." They even threatened to arrest him, though Bieber and his team have been cooperating with police orders.

This marks the second time this week that Bieber ignited fan fury. A source tells that when the pop star and his stylist and "swagger coach" Ryan Good visited Starbucks several days ago, the singer was "trapped" inside the coffee shop as fans "demanded to meet Bieber."

"The fans are nuts over there," the insider adds.

Bieber, who in town for his stylist's birthday party, is no stranger to scary run-ins. In November 2009, the "Baby" singer was forced to cancel his appearance at a Long Island mall after 3,000 fans created a mob-like situation.

Five minor injuries were reported, and Island Def Jam vice president James Roppo was arrested and charged with assault, endangering the welfare of a child, obstructing government administration, reckless endangerment and criminal nuisance; he pleaded not guilty on all charges.

Bieber's manager, Scott "Scooter" Braun, was arrested on one count of second-degree reckless endangerment and one count of second-degree criminal nuisance related to the incident; he pleaded not guilty to both charges and was subsequently released.