Thousands show up for a rally in led by ultra-popular opposition frontman Thanathorn Juangroongruangkit and his Future Forward Party (FFP) executives. (AFP)

Thai police arrested a suspect behind an “inappropriate” photo posted on Facebook from an anti-government rally, a minister said Wednesday, as the offending image went viral days after the largest protest since March’s elections.

Downtown Bangkok saw thousands turn out Saturday for a rally led by ultra-popular opposition frontman Thanathorn Juangroongruangkit and his Future Forward Party (FFP) executives.

Carrying posters denouncing “dictatorship” and calling for prime minister Prayut Chan-O-Cha to “get out”, the protesters rallied peacefully for an hour.

The event drew the ire of authorities, however, with Digital Economy and Society Minister Buddhipongse Punnakanta singling out an “inappropriate post of a picture that has upset all Thais”.

The photo shared was snapped during the rally, displaying a protester holding a profanity-laced sign decrying “dictatorship”.

A mural of the revered late King Bhumibol Adulyadej appeared in the background.

Thailand has some of the world’s harshest lese majeste laws, but authorities have not specified what the suspect was charged with.

Buddhipongse said he had conducted investigations before the suspect’s arrest, but refused to give any further details.

“No one can tolerate this kind of thing,” he said in a Facebook post.

The monarchy sits at the apex of Thai society, buttressed by the powerful military — which has staged more than a dozen coups since 1932 — and the business elite.

The upstart FFP has proven a thorn in the side of the military-aligned establishment since it rode a youth-propelled wave to become the country’s third-largest party in this year’s elections.

Since then it has called for an end to military conscription and for the defence budget — one of the highest in the region — to be slashed.

But it has also come under fire for voicing political objection to a royal command, unprecedented in Thailand.

Authorities have threatened legal moves against Thanathorn and FFP for not obtaining permission for Saturday’s rally, although police said Wednesday no charges had been filed.