Police closed off a block near the popular tourist spot around 5 p.m. Saturday so investigators could take a closer look at the device.
It turned out to be a replica. The area was reopened around 6:15 p.m.
The juggler was detained, but a police spokesman did not know if he had been arrested.
Police did not identify the man. It was not known if he was a regular performer in the area, or had used the replica grenade in previous juggling acts.
