One person described the scene outside a coffee shop where two city inspectors and two policemen were stopping many foreigners.

"They were thoroughly searching tourists and their belongings, asking them to produce passports," the person told the Bangkok Post. "Many of those unfortunate souls were then detained and I saw them paying money to the officials ... one girl was reduced to tears and was obviously very upset."

A blogger who attempted to get first-hand evidence of an apparent shakedown was chased by officers and had to hide in a mall.

Mark Kent, the British ambassador in Thailand, tweeted on Thursday that he has raised the issue with Thailand's tourism authorities.

The reports come at a time when the country's tourist industry is struggling to recover from a military takeover in May following months of political upheaval. Martial law giving authorities wide powers remains in force, including the banning of gatherings of more than five people throughout the country. Officials have the power to search any one or any place.