A bottle-nosed whale was spotted in the Thames on Friday morning. The mammal got as far upstream as Chelsea.
"I saw it blow, it was a spout of water which sparkled in the air," said Tom Howard-Vyne, head of communications at the London Eye.
The RNLI sent a lifeboat to monitor the whale. Port of London Authority and London Fire Brigade vessels have also been in attendance.
The BBC and Sky News have helicopters overhead broadcasting live pictures.
A PR firm based in Millbank Tower has set up a blog to track the whale's progress.