Wearing shirts in the queen’s color – sky-blue – and emblazoned with “Bike for Mom”, the cyclists paraded through streets closed to traffic.
Many Thais outside Bagkok were expected to do the same.
Queen Sirikit – who turned 83 four days ago – is in very frail health, like King Bhumibol, and will not be seen.
Commentators say the 27-mile cycle is the first event to be publicly led by Prince Maha Vajiralongkorn, who is 63.
About 300,000 people registered to join the ride.
Prince Maha Vajiralongkorn was followed by other well-known figures including PM Prayuth Chan-ocha and his cabinet.
Cycling in Thailand 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.
It is the rare appearance at such a high-profile public event by Prince Maha Vajiralongkorn which will be noted by royal-watchers.
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.
King Bhumibol 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.
Organizers 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.
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