Police were unable to control the turbulent clash between red-shirt and yellow-shirt supporters in Bangkok on Tuesday because the brawling was spread out and not just in one spot, Crime Suppression Division (CSD) deputy chief Prasopchok Prommoon said on Wednesday.

"Although officers tried to separate the two groups and told them to stay within designated areas, the protesters still spread out to different places and confronted each other," Pol Col Prasopchoke said.

"I believe that even if we had reinforcements there would still have been clashes because they were continually trying to aggravate the situation."

The colour-coded protesters gathered after police summoned for questioning Manasnant Nookham, a former teacher who publicly accused key red-shirt supporter Darunee Kritbunyalai of defaming the royal institution last month.

The 31-year-old former teacher was summonsed to report for questioning on Friday, but her lawyer went to the CSD on Wednesday and told police she had gone abroad and he had not been able to contact her yet.

Her reporting appointment was then moved to Oct 29.