Buildings were evacuated, a street was cordoned off and a bomb disposal team called in after workmen spotted a suspicious object.
But the dangerous-looking weapon turned out to be the Holy Hand Grenade of Antioch, made famous in the 1975 film Monty Python And The Holy Grail. Police and a fire crew were first on the scene in Shoreditch, east London, when water company workers found a copy of the film prop under a fire hydrant cover.
They evacuated a pub and another building in Tabernacle Street, while office staff in another building were stopped from leaving.
But when the bomb squad arrived, they quickly established there was no danger and the street was declared safe. In the film, the grenade was used to slaughter a killer rabbit. Python actor Michael Palin said: "Oh Lord. Bless this hand grenade, that with it thou mayest blow thine enemies to tiny bits, in thy mercy."
Alberto Romanelli, who owns the Windmill pub nearby, said the police action in ordering his pub to be evacuated had been as ridiculous as the film scene. "They evacuated the pub while they were doing X-rays and stuff," he said. "It all lasted about 45 minutes before they decided it was nothing - which I thought was pretty obvious from the start. I lost a good hour's worth of business."
An Islington police spokeswoman said: "There was no danger to the public. The device is believed to be an object known as a Holy Hand Grenade."