A bloody scuffle in Phnom Penh on Saturday left scores injured after protesting bus drivers and their supporters were brutally set upon by a rival association armed with sticks, metal bars and hammers.

Police arrested two protesters – former Capitol bus driver Norn Van, and Cambodia Labour Confederation member Ros Sipha – but none of the alleged attackers.

A statement signed by 34 civil society organisations blasted the police for ignoring the beatings and called “for the immediate release of the arbitrarily detained victims and … a prompt, independent and transparent investigation into today’s events”.

On Saturday morning, a total of about 50 former drivers of the Capitol Bus Company and their supporters from the Cambodia Labour Confederation gathered in front of the Capitol office near O’Russey Market to protest the firing of 45 drivers for trying to start a union.

But after the demonstrators attempted to block a bus from leaving, they were rushed en masse by members of the Cambodia for Confederation Development Association clothed in black and wearing helmets.

Drivers from the CCDA, which represents tuk-tuk drivers and motodops, but is widely perceived as acting at the behest of employers, can be seen on video viciously beating the protesters, with one slamming a hammer twice into the back of a cowering bus driver.

Soon after, police pushed back the protesters and arrested Van and Sipha.

Rights group Licadho said that 14 people were injured in the violence, including one policeman, while Sipha was allegedly beaten by police.

Prampi Makara district deputy police chief Krek Sakarin said the two men were sent to court over the violence yesterday and hung up the phone.

Moeun Tola, executive director of rights group Central, who was at the scene of the attack, said the CCDA’s beatings seemed planned in advance.

“If you plan for a peaceful protest then why do you have helmets? Some had walkie-talkies and hammers . . . They don’t look like tuk-tuk drivers.”

Tola added that the CCDA was “not independent”, and was rumoured to be funded by Capitol and authorities.

“I feel like the authorities use these anarchic groups to promote violence and stop people from exercising their right to freedom of expression,” he said.

The CCDA has a long history of demonstrating against civil society groups. In 2012, it claimed that leaders of the Boeung Kak community beat them and held protests in front of Licadho’s offices.

Several motodops during a later protest in front of the Cambodian Center for Human Rights in 2013 said in interviews that they were given food or money to attend and had no idea what they were protesting about.

But CCDA president E Sophors said yesterday that Saturday’s violence only erupted because the protesters had disrupted their agreement with Capitol to provide tuk-tuks and motodops for passengers at the office.

“We would not hit the protesters without reason, but those protesters blocked the bus and sealed the road in front of the company and affected our business,” he said, claiming that 20 CCDA members were also injured.

Capitol issued a statement accusing the CLC and the drivers of sparking the violence, claiming that they threw stones at police and stabbed a tuk-tuk driver.

“The company is the victim,” the statement said.

But CLC president Ath Thorn said it was clear from video that the violence did not come from the protesters’ side, and warned of large demonstrations if the two arrestees were not released.

Capitol bus driver representative Hin Moeun said that drivers had agreed to call off protests in front of the office following the violence.

Additional reporting by Charles Rollet