"For anyone twitching the #BELUGA its been feeding around the barges (see last tweet for location) for the last hour and hasn't moved more than 200m in either direction".
Another said: "Almost unbelievably, it would seem that there is now a beluga whale in the Thames".
Conservationists have spent the night keeping an eye on a beluga whale in the River Thames.
Experts have urged the public to stay away from the mammal, which is thought to be hunting for food, and watch it from shore instead of going out in boats.
"We would urge that the whale is given space and disturbance is kept to a minimum", he said.
Tanya Ferry, environment manager at the Port of London Authority, said it was unclear what the whale could eat.
However rescue teams were on standby in case the animal got into danger.
Lucy Babey, head of science at Orca, a marine conservation charity, said: "This is the most southerly recording of a beluga in the UK".
"We certainly don't want people trying to rescue it", she added.
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.
It said it was ready to provide help to the whale if asked to do so by other agencies.
A bottle-nosed whale became stranded in the Thames for two days in 2006, but died from convulsions as it was being rescued. They can be found in the waters around Russia, Alaska, Canada, West Greenland, and Svalbard.