The rival colour-coded protesters gathered after police summoned for questioning a former female teacher who publicly accused red-shirt key supporter Darunee Kritbunyalai of defaming the royal institution last month.
The woman was told to report for questioning on Friday, but her lawyer was to go and see the CSD on Tuesday. Her reporting appointment was postponed to Oct 29 following today's clash.
Bangkok Post reporter Voranai Vanijaka, who was at the rally scene, said there were nearly 200 red-shirts and about 50 yellow-shirts gathering outside the compound on Tuesday afternoon. Many of them appeared to be over 40 years old.
He said the red-shirts were waving flags, showing middle fingers and taunting continually, while the outnumbered opposition looked nervous and scared.
At times, the red-shirt protesters tried to cross to the yellow-shirt's side, but then retreated.
The reporter said there were no weapons. Both sides were throwing plastic bottles at each other. An older yellow-shirt woman was reported to be swinging an umbrella at her opponents.
One of the protest groups held a sign that read 'We Love the King' while another group held a sign that said 'We Love the Prince'.
Police tried to calm down the situation by urging them to go home and saying "We are all Thais". No arrests were made.
Mr Voranai reported that many of the yellow-shirts were not happy with the police after they failed to control the situation.
"Let's just give Thailand to the red-shirts because the police can't do anything," shouted one of the yellow-shirts.
(story continues below)
(Photos by Surapol Promsaka na Sakolnakorn)
As the situation gradully escalated, Facebook fan pages for both sides were calling out for their supporters to join the rally, with both sides claiming that they were outnumbered.
Mark Fan Club Facebook page posted pictures of a red-shirt Facebook page's posting where they claimed that they were outnumbered by the yellow-shirts by five to two.
They claimed that there were about 500 yellow-shirt supporters against only 200 red-shirts.
Several yellow-shirt fan pages, such as the Uprising People, claimed that their supporters were unarmed and outnumbered by the growing number of red-shirt supporters.
The fierce confrontation ended around 3pm. The winning side appeared to be the red-shirts as the yellow-shirts retreated to nearby areas, according to the Bangkok Post reporter.
Reports said a total of five people were injured in the fight. They were two red-shirts, two yellow-shirts and a news reporter.
A red-shirt's pickup truck was damaged during the clash, the windscreen smashed.
Thawee Surarittikul, a political analyst at Sukhothai Thammathirat Open University, expressed his concern about the political situation following the clash on Tuesday.
"The situation was contained, but what will happen if the situation goes out of control next time.
"Both sides are waiting for an issue which could be a trigger point leading to a bigger protest," Mr Thawee said.
"Any controversial or sensitive issue picked up on by either side could possibly spark a 'swarming effect'," he said.
A video clip, uploaded to Youtube by Prainn Rakthai, shows the clash between red-shirt and yellow-shirt protesters in front of the CSD offices on Phahon Yothin road on Sept 25, 2012.