More than 30,000 people are expected to gather by the banks of the River Thames for the opening of the Henley Royal Regatta.
Organisers expect about 250,000 people over the next five days.
The annual rowing competition takes place over a stretch of the Thames, just under a mile-and-a-half in length, on the border of Berkshire and Oxfordshire.
Top British Olympic rowers Matthew Pinsent and James Cracknell are expected to take part again in the five-day event.
The regatta operates a knock-out draw with only two boats racing in each heat, which entails the organisation of up to 100 races on some of the
Organisers are expecting a total of about 250,000 people to attend the regatta.
In its 154-year history the regatta has often been honoured by visits of members of the Royal Family, of which the most recent was The Princess Royal in 1999.