Thousands of Thai cyclists have joined Crown Prince Maha Vajiralongkorn in a ride through Bangkok in honour of his mother, Queen Sirikit.
Wearing shirts in the queen’s colour – sky-blue – and emblazoned with “Bike for Mom”, the cyclists paraded through streets closed to traffic.
Many Thais outside the capital were expected to do the same.
The queen – who turned 83 four days ago – is in very frail health, like the king, and will not be seen.
Commentators say the 43km (27-mile) cycle is the first event to be publicly led by the prince, who is 63.
About 300,000 people registered to join the ride.
The crown prince was followed by other well-known figures including Prime Minister Prayuth Chan-ocha and his cabinet.
With sweltering heat and unforgiving traffic Bangkok is an unpromising setting for a cycling event, says the BBC’s Jonathan Head in Bangkok.
But against the odds, cycling has taken off as a hobby in recent years, and on Sunday the heir to the throne tapped into that craze.
The prime minister said he hoped it would unite a country which is still divided over last year’s military coup.
But it is the rare appearance at such a high-profile public event by the 63-year-old crown prince which will be noted by royal-watchers, our correspondent says.
Many Thais are anxious about what happens after the death of 86-year-old King Bhumibol Adulyadej, the world’s longest-reigning monarch, whose health is very frail.