The photo shared by Kamron Petprayoon on his Facebook with 7,600 followers on Jan. 12 received 1,300 likes and 2,100 shares as of Wednesday.
A Facebook user wrote "This is a holy place. You should respect it."
Another complained "Inappropriate, very bad. There aren't any officials around there?"
Another wrote "How dare they? They should know that it is not a place that allows sunbathing around like this."
Many Thai netizens expressed their disappointment, saying that the tourists were "impolite and disrespectful to Thai culture."
Located at the heart of Bangkok, The Grand Palace is considered as the symbol of Thailand and Thai royal family. Now, the place is used for hosting royal ceremonies and welcoming the king's guests, state guests, and other foreign dignitaries.
Sanam Luang is traditionally used for royal ceremonies, festivals and events of national significance.
The Thai royal family is highly respected by Thai people and no insults are allowed.
Some netizens also blamed the officials for not putting up signs in the area to remind foreigners that sunbathing should be banned in Sanam Luang.
A netizen said it should be the responsibility of Thai officials to erect signs to tell foreign tourists the area is a revered place.
The Phra Nakhon District Office in Bangkok said police were immediately dispatched to the scene after receiving a complaint from locals about the sunbathing.
The two foreign tourists were warned and agreed to follow the instructions, police said as cited by Nation Thailand.
Officials also promised to put information signs in the area to prevent future incidents.
On Sunday, a similar incident also happened at Chiang Mai’s Wat Chiang Man temple, when two women were found sunbathing in the shrine’s premises, took photos and shared them on social media.