A whale swam up the River Thames in central London on Friday, past Parliament and Big Ben.
Rows of puzzled spectators on the river banks watched the mammal navigate between boats and under bridges, while divers followed its movements, promising to help it if it became stranded.
"I saw it blow. It was a spout of water which sparkled in the air," witness Tom Howard-Vyne said. "It was an amazing sight."
It was the first time this type of whale, a northern bottlenose whale, has been sighted in the River Thames since the Natural History Museum began recording such sightings in 1913, museum zoologist Richard Sabian said.