The Metropolitan Police Marine Policing Unit, which is following the group of mammals, tweeted that they "seem happy enough".
Initially the river patrol team, which tweets as @MPSonthewater, reported a sighting of a dolphin at Tower Bridge.
But further updates revealed the creatures to be harbour porpoises, which usually stick to coastal areas and river estuaries, but are known to venture further upstream.
The Marine Policing Unit tweeted: "Marine 2 are following a pod of about 5 harbour porpoises in the Lambeth area of the river."
It later added: "The porpoises seem happy enough. They are surfacing regularly and are staying in the Westminster area"
