Many on board the London Duck Tours boat jumped into the water as flames took hold and thick black smoke billowed from the amphibious vessel.
A woman and a child were taken to hospital with suspected smoke inhalation.Further tourists were treated at the scene of the fire, opposite the Houses of Parliament.A police source said the nature of the injuries "could have been a lot worse".
A Scotland Yard spokesman said: "A number of people were reported to be in the water at the time. Several Metropolitan Police units attended and the police helicopter."A number of people were recovered from the water. All 28 passengers and two crew members have been accounted for."
More top news
A drunk man who narrowly escaped death when he lay down between train tracks in Harlow has been given a 12-month community order.
A man from north London has been charged with the manslaughter of Louella Eve Fletcher-Michie, who died at the Bestival music festival.
The mother of Ellie Butler - the six-year-old murdered by her father - has lost a bid to challenge her conviction.