Vajiralongkorn became king after the 2016 death of his father, King Bhumibol Adulyadej, who reigned for seven decades, but it wasn't until his formal coronation on Saturday that he became a monarch with full regal powers based on the nation's traditions.
Thailand has had a constitutional monarchy since 1932, when a revolution ended absolute rule by kings. The country's monarchs are still regarded as almost divine and seen as a unifying presence in a country that has suffered regular political instability as it rotates between elected governments and military rule.
The king and other top members of the royal family are protected by one of the world's strictest lese majeste laws, which makes criticism punishable by up to 15 years in prison. People began lining up early Monday so they could be near the balcony where the king was to appear. Large video screens were placed nearby so those unable to make their way to the front could watch. As the crowd waited in the blazing tropical heat, some broke into cheers whenever a passing cloud blocked the sun.
"I'm glad to be born as a Thai," said Tipparat Aiyawan, who works in Bangkok. "I want every child to know that Thailand has had independence for centuries and has survived because our kings are the soul of our nation."
Vajiralongkorn, arriving late, was greeted inside the royal throne hall by dignitaries from business and religious circles. When he and his wife, Queen Suthida, a general in the palace guard whom he married just last Wednesday, stepped outside onto the balcony, a mighty cheer erupted from the crowd, most dressed in yellow, the color representing the monarch's birthday. Nearby, artillery was fired in salute.
For about two minutes the king stood with the queen, raising his right hand in greeting before sitting down. Below the balcony, Prime Minister Prayuth Chan-ocha read a short speech pledging loyalty to the monarch.
The king stood up, stepped in front of a microphone and read from some papers. He said he and his queen were delighted with the goodwill extended by the people, and their good wishes left him "moved."
"May your unity in extending good wishes to me on this occasion become a good sign that everyone and every party will work for the prosperity of our nation," he said. The king's 14-year-old son, Prince Dipangkorn Rasmijoti, and two daughters from previous marriages then stepped forward to join them on the balcony and wave to the crowd. Princess Sirivannavari Nariratana, a fashion designer who is the younger of the king's daughters, took photos with her cellphone and waved enthusiastically.
The king and queen then departed for a quick audience with foreign diplomats. Vajiralongkorn is also known as King Rama X because he is the tenth king in the Chakri dynasty, which began in 1782. His coronation has involved a series of elaborate, centuries-old rituals rooted in Buddhist and Brahmanic traditions.
A final coronation celebration is planned for October, when there will be a royal barge procession on Bangkok's Chao Phraya River.
Associated Press journalists Gemunu Amarasinghe, Tassanee Vejpongsa and Preeyapa T. Khunsong contributed to this report.