A man swinging a samurai sword fought an imaginary duel Friday on the glass spiral staircase of the bustling Apple Store on Fifth Avenue as frightened tourists looked on.

Video posted to Instagram shows the swordsman walking up the staircase, and then sitting on it while performing pretend lunges and parries for his startled audience at around 3:55 p.m.

He was quickly tackled by Apple security and turned over to police, who took him to Bellevue Hospital for a psychiatric examination.

Nancy Birnbaum, a lawyer who lives in the area, was browsing at the store when she saw the man on the staircase suddenly unsheathe his weapon.

"Usually the staircase is mobbed with people, but this time it wasn't. There was a guy in the mid part of the staircase kneeling," Birnbaum told The Post.

"His face looked a little contorted and twisted," she added. "I thought he was doing some sort of performance art, but then he pulls out this sword with a huge 2-foot-long blade and it was dead obvious that it was a real sword.

"I don't know if he is from ISIS or wherever the heck he is from but I thought this is the beginning of some sort of horrible headline tomorrow"

- witness

"I've never been so terrified in my entire life."

Birnbaum said that she feared the worst.

"I don't know if he is from ISIS or wherever the heck he is from but I thought this is the beginning of some sort of horrible headline tomorrow," she said.

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