A rare sea mammal - initially thought to be a dolphin - has sparked alarm after it was spotted by Londoners in the River Thames - more than 40 miles inland.
Stunned witnesses have posted videos of the big fish alone in the capital on social media.
The RSPCA has confirmed it is monitoring the mammal, which has now been identified as a harbour porpoise, and police have called for it to be left alone.
Among those who saw it was Amy Clayfield, who uploaded a video of the porpoise on Twitter, alongside the caption: "There's a dolphin in the Thames!"
It was first seen yesterday close to Putney, and another video was posted by musician Jack Lawrence Brown, who wrote: "Dolphin in the Thames this evening around Hammersmith, the strangest sight."
Today people have been crowding on the river bank hoping to catch a view of the unusual animal.
Bankside pub the Bell and Crown wrote: "Everyone is riverside today, trying to catch a glimpse of the dolphin in the Thames."
And this afternoon Alastair Hilton wrote: "I'm in chiswick, strand on the green, watching a dolphin swimming in the Thames! I kid you not!"
The RSPCA has confirmed it is monitoring the porpoise alongside British Divers Marine Life Rescue, which helps marine animals in distress.
And the Metropolitan Police Marine Policing Unit wrote today: "We're aware a harbour porpoise has been spotted up at Kew. It's perfectly fine so please don't approach it as it's likely to beach itself."