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.
Analysis: Jonathan Head, BBC News, Bangkok
Thai officials often quietly remind journalists that Crown Prince Vajiralongkorn is the undisputed heir to the throne, citing the 1924 palace succession law and his investiture in 1972.
The harsh lese majeste law prevents any open discussion of the succession, but that has not stopped clandestine gossip in Thailand about "alternative scenarios", usually involving the crown prince's popular sister Princess Sirindhorn.
This rare appearance at such a high-profile, public event by the Crown Prince - heavily promoted by the military government - will send out a clear signal that he has the backing of the armed forces, and that he can be a "man of the people", like his father.
It also shows him as a loving and dutiful son. With King Bhumibol in increasingly frail health after 69 years on the throne, the military is pre-occupied with managing an orderly succession; indeed that may have been uppermost in the generals' minds when they launched last year's coup.
On Sunday, Thais saw the heir to the throne, usually a remote figure, leading tens of thousands of eager cyclists in Bangkok, all paying tribute to his mother. Few will miss the symbolic importance of such an event.
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.
He is widely revered, but observers say the heir apparent does not command similar affection.
This event is a chance to improve his public image and promote national unity.
Organisers of the cycle ride are also attempting to set a new world record for the greatest number of people cycling at once.
The record is currently held by Taiwan, with 72,919.