Youth activists protesting against laws forbidding insult to Thailand's powerful king briefly clashed with police on Saturday after draping Bangkok's Democracy Monument in red cloth.

Protesters threw paint at police and several small bangs were heard during a standoff near a city shrine after the demonstration had moved from Democracy Monument and the main leaders had called for it to disperse.

Activists said the red cloth on the monument represented the blood of fighters for democracy.

Police deputy spokesman Kissana Pattanacharoen said more than 20 police officers were injured in the clashes and seven or eight people were detained for questioning. He also said at least one firecracker exploded at the scene.

The demonstration was in reaction to this week's arrests of four leading protest figures under charges of insulting the monarchy in mass anti-government demonstrations last year.