At 8am, Thais from around the country arrived to celebrate the 103rd anniversary of the tri-coloured flag, which was instituted by King Vajiravudh (Rama VI) in 1917.
The military-backed government founded flag day with much fanfare to mark its centenary in 2017. The occasion is not a national holiday, though.
On Monday, people at the Lumpini Rama VI monument sang the national anthem and raised the flag to the top of the pole.