Conservationists said they hoped the beluga's natural instincts would soon kick in and it would head out of the estuary and back north.
Yes, a beluga whale was spotted splashing around in Britain's River Thames, and Brits can't quite believe it.
Beluga whales are usually found in the Arctic Ocean, with males measuring as long as 5.5 metres and can also weigh up to 1,600kg.
By Tuesday lunchtime, photographers were lining the banks of the Thames, as were locals and others, and the BBC had launched its own live-stream of the creature, with some folks giving themselves the afternoon off work just to watch it.
Officials say they're monitoring the situation from afar too.
Beluga whales were last spotted in the United Kingdom three years ago off the coast of Northumberland and Northern Ireland, but sightings were "extremely rare", spokeswoman Julia Cable said.
Benny, as it has now been named, appeared to be "swimming strongly" and feeding in the estuary but concerns for the whale's health have continued.
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He urged people to give the whale "space and minimise disturbance".
They are common to many regions of Alaska, as well as Russia, Canada, and Greenland.
A female northern bottlenose whale was discovered swimming in the river in central London, going past the Houses of Parliament.
The whales were nicknamed "canaries of the sea" by early whalers due to their squeaks and squawks.
"Beluga whales are not usually seen in British waters, so it's very rare", spokeswoman for the organization Julia Cable said.
Beluga calls variously resemble a cork being prized from a bottle or a creaking door, along with sounds described as clicks, squeaks, chirps, bleats, moans, groans, and whistles.