Julian Assange has claimed a package containing a "threat" and white substance was sent to him at the Ecuadorian Embassy.
The item, which has been deemed "non-suspicious" by police, was addressed to Mr Assange, he said in a tweet.
It comes on the day a court ruled an arrest warrant against the WikiLeaks founder is still valid despite Swedish authorities ending an investigation against him.
The Metropolitan Police Service (MPS) said the incident was "stood down" after the package was assessed.
Mr Assange wrote: "I can confirm that a package containing an unknown white powdery substance and a threat was addressed to my name.
"It was handed to UK diplomatic police. I understand they are performing toxicology and forensics."
MPS Kensington and Chelsea tweeted: "Police attended the Ecuadorian Embassy in SW1 following reports of a small suspicious package.
"The item was assessed by specialist officers and deemed to be non-suspicious.
"There are no injuries and this incident has now been stood down."