Daniel Parmenter, 44, is on trial at the Old Bailey, accused of making a bomb hoax on 22 November last year.
The US Embassy, in Nine Elms, south London, went into lockdown, and bomb disposal experts carried out a controlled explosion, after a bag containing wires and an iPad was left outside, the court heard.
Mr Parmenter has denied intending to cause alarm. He claimed in a formal police interview that it was an “art installation”, the court was told.
Opening his trial on Monday, prosecutor Lucy Organ told how Mr Parmenter had left the suspicious item in an alleyway by the embassy around 6am.
A civilian embassy guard noticed what she thought looked like a bomb on a routine patrol, and “panicked”. Ms Organ said. The guard raised the alarm.
Jurors were told a police officer on duty at the embassy walked down the alleyway and saw a number of objects, including photo frames and a metal tray with a skull and crossbones on it.
Ms Organ said: “Next to all of this on the floor next to the wall was what appeared to be an IED (improvised explosive device), it was an old-style iPad with a key pad and firework wires all taped.”
There was also a box of dates marked for the attention of the US Navy with the words “do not X-ray, please inspect. Radiation. Hazard”.
Mr Parmenter was identified following an examination of CCTV from the area. He was arrested at the home he shares with his mother, wearing the same hat and jumper he had on in the CCTV footage, two days later.
On being arrested on suspicion of planting a device at the US Embassy, the defendant replied “I did” even before the officer had finished speaking, the court was told.
Mr Parmenter denied meaning any harm, saying: “It is basically a form of slightly sophisticated graffiti art of the non-vandal type.”
The defendant’s basement flat, described as a “hoarder’s property”, was searched, and police found what appeared to be a cross between a petrol bomb and a pipe bomb at the property, the court heard.
Explosives officers determined that it was not a viable device.
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Ms Organ told jurors that the defendant did not deny leaving the objects on the perimeter wall of the US Embassy.
The issue in the trial would be whether Parmenter intended others to believe the device was likely to explode or ignite, and cause personal injury or damage to property, she said.
Mr Parmenter, of Kildare Terrace, Bayswater, west London, has denied making a bomb hoax.
The trial continues.
