The People's Alliance for Democracy (PAD) will tomorrow file a complaint with the National Police Commissioner, General Kowit Wattana, and submit a videotape that allegedly shows a high-ranking police officer ordering two men to assault protesters.

Suriyasai Katasila, coordinator of the anti-government group, said he had a video recording showing Colonel Rittirong Thepchan, a superintendent of Metropolitan Police Bureau 6, ordering Charan Chong-on and Chaiyasith Lamoh to assault anti-Thaksin protesters at Bangkok's Central World Plaza on Tuesday.

Although Charan and Chaiyasith were not police officers, they had close ties with Rittirong and police allowed the two to assault protesters, he claimed.

Suriyasai suspected the men were police helpers who were used to suppress anti-government demonstrators.

He claimed the surrendering of the two to police was just a set-up aimed at reducing public pressure.

He said police officers who allowed these two men to interfere in their work must be punished. A group of young men who attacked and seriously injured an elderly protester on Tuesday must also be identified and punished for the crime, he said.

Thai Rak Thai Party spokesman Sita Divari said the violence was a set-up by government opponents determined to oust premier Thaksin Shinawatra.

"The anti-government group knows that if there is violence, the government's image will be adversely affected,'' he said.

Caretaker Deputy Prime Minister and Justice Minister Chidchai Vanasatidya said he would ensure a speedy investigation into the assaults and would take legal action against anyone involved. He denied that police would discriminate against certain groups, saying they cannot because they are under media scrutiny.

Metropolitan Police chief Lt-General Wirot Chantarangsi yesterday ordered an investigation into the complaint that police allowed a group of men to assault protesters.

Wirot appointed his deputy, Maj-General Kamol Kaewsuwan, to head the panel probing the allegation.

Meanwhile, Privy Council President Gen Prem Tinsulanonda has expressed concern over the heightening violence between supporters and critics of Thaksin, one of Prem's close aides said yesterday. He was worried that confrontations between the groups could worsen, according to the source, who requested anonymity.

In a related development, the Pracharaj Party yesterday denounced the use of force against protesters. In a statement, the party called on Thaksin to refrain from showing aggression towards his critics in order to ease the volatile atmosphere.