Hundreds of people gathered at the Ananda Samakhom Throne Hall on Monday for the ceremony as the royal anthem played for the Thai king and queen.
The foreign guests arrived in processions, paid their respects to Bhumibol, the world's longest serving monarch, and greeted his family members one by one with handshakes.
The dignitaries were to join the king later at the Royal Navy Yard along the Chao Phraya river for a procession of regal barges.
More than 2,000 oarsmen will row 52 ornate boats down the river in a procession 1,200 metres long and 90 metres wide, in one of the most widely anticipated events of the five-day celebration honouring the king.
At Bangkok's airports, attendants in white tunics held purple parasols to shield royals, while women in colourful silk clothes strewed flower petals on a red carpet as they arrived.
Crown Prince Vajiralongkorn (L)
escorts Japan's Emperor Akihito
Thai officials have made elaborate preparations to ensure the celebrations go smoothly, including plucking stray dogs from Bangkok's streets, and closing off major streets to facilitate transportation for the VIPs.
Bangkok residents expect major traffic snarls.
The king has reigned through dozens of governments, democratic and dictatorial.
Bhumibol, a constitutional monarch with limited powers, has used his prestige during political emergencies to pressure opposing parties to compromise, and is credited with helping to keep Thailand more stable than many of its Southeast Asian neighbours.
Many Thais are counting on him to pull the country through its current political situation, which has left the country with no functioning legislature and only a caretaker government after a divisive, inconclusive election.