TV news helicopters filmed the white whale from the air as officials asked the public not to get too close to the animal. The Whale and Dolphin Conservation group said that beluga whales are identified by distinctive white markings and are typically found farther north.
"Beluga whales inhabit cold, arctic waters off Greenland, Svalbard and in the Barents Sea," the group said in a statement . "There have only been around 20 sightings of beluga whales off the U.K. coast previously, but these have occurred off Northumberland, Northern Ireland and Scotland."
The RSPCA animal welfare group said that it's "working with other agencies to monitor the situation" and sent researchers to the scene. It says it is ready to provide help to the whale if asked to do so by other agencies.
The River Thames, one of the longest rivers in England, runs through several major cities and towns, including London, Oxford and Windsor.