Thailand's army chief threatened Monday to "intensify law enforcement" against anti-coup protesters who have pledged to rally in Bangkok for a fourth day over the military's seizure of power.

General Prayut Chan-O-Cha, who was on Monday given royal endorsement to govern the country, said protesters found in breach of martial law could be tried in military courts, in his first press conference as de facto Thai leader.

If rallies continue "I will intensify law enforcement", he warned, adding "when the situation relaxes we can relax those measures".

Asked how long the junta would be in power he said the army would take control "until the situation is resolved" refusing to be drawn on whether he would eventually become prime minister.

As coup-maker Prayut has swept through orders including curbing media freedoms and banning political gatherings of more than five people.

Anti-coup protesters have held several seemingly spontaneous rallies in downtown Bangkok since last Friday.

They have swiftly grown in size, drawing around one thousand angry protesters on Sunday in a rally that rolled through Bangkok's commercial heart.

Demonstrators, many holding anti-coup placards, have scuffled with soldiers and several have been detained, raising fears of a wider crackdown if they continue to breach martial law.

They are due to hold another rally on Monday afternoon.