Six participants in the Diamond Jubilee river pageant were taken to hospital after becoming cold and wet, the ambulance service has said.
Another 40 people were treated by medics on the banks of the Thames after the parade was beset by heavy downpours
By the time boats reached Tower Bridge, many of their occupants were shivering. A London Ambulance Service spokesman said.
Their conditions are not said to be serious. Temperatures remained around 12C during the parade which was hit by drizzle from 3.30pm.