Thousands of well-wishers lined the banks of Bangkok's Chao Phraya River Sunday to watch King Maha Vajiralongkorn ride a glittering royal barge procession to mark his 72nd birthday.

A flotilla of 52 ornately decorated boats, paddled by more than 2,000 oarsmen decked out in scarlet and gold, carried the king and Queen Suthida in formation through the heart of the Thai capital to a Buddhist ceremony at Wat Arun, the city's ancient Temple of Dawn.

The king, officially regarded as semi-divine but who came in for unprecedented criticism in street protests in 2020 and 2021, took his place on a century-old royal barge known as the "Golden Swan" to deliver robes to monks in a ceremony marking the end of Buddhist Lent.

Royal barge processions date back hundreds of years, but are held rarely, saved for the most significant occasions — most recently, the king's coronation in 2019.

During the 70-year reign of the previous king, Bhumibol Adulyadej, only 16 barge processions were held.

King Vajiralongkorn turned 72 in July, completing his "sixth cycle" in the 12-year astrological calendar — a milestone regarded by Thais as important and auspicious.

Normally the intricately ornamented barges — their prows decorated with garudas, nagas and other mythical creatures from Buddhist and Hindu mythology — are kept in a museum.

But on days of national importance, navy oarsmen in sarongs, red tunics and traditional hats propel them through the water to the banging of drums, as perfectly coordinated golden paddles break the waters.

Only four of the barges are actually deemed "royal," while the others are officially royal escort vessels.

The barge procession dates back to Thailand's 1350-1767 Ayutthaya period. When Bangkok was built more than 250 years ago, kings used the boats to travel through the capital's network of canals.

As Thailand modernized, the barges fell out of use, but king Bhumibol revived the tradition in 1957 to celebrate the 25th century of the Buddhist era.