Thousands of cyclists are being led through the streets of Bangkok by Thailand's crown prince on a 43-kilometer (26-mile) tribute to his mother, Queen Sirikit, to mark her 83rd birthday.

Sunday's event, led by 62-year-old Crown Prince Vajiralongkorn, was also an exercise in palace public relations at a time of concern about the royal succession in Thailand.

The country's revered monarch, 87-year-old King Bhumibol Adulyadej, and his wife, Queen Sirikit, are both in declining health and have been absent from public life for years. Their only son and heir apparent does not command the same respect and affection as his father.

Sunday's bike ride was an apparent effort to rehabilitate the image of the crown prince, who is usually seen at formal events and rarely mingles with the public.