It should surprise no one that some if not all of the violent pro-Mubarak forces are plain clothes police officers.
The Guardian notes:
Sharif Kouddous, a prolific Egyptian tweeter and blogger in Cairo, describes "a brutal and coordinated campaign of violence" by the Mubarak regime, in an article posted on Democracy Now's website:
"Suddenly, rocks started falling out of the sky," said Ismail Naguib, a witness at the scene. "Rocks were flying everywhere. Everywhere." Many people were hit. Some were badly cut, others had arms and legs broken. The mob then charged in; some rode on horseback and camels, trampling and beating people. Groups of them gathered on rooftops around Tahrir and continued to pelt people with rocks.
"It's a massacre," said Selma al-Tarzi as the attack was ongoing. "They have knives, they are throwing molotov bombs, they are burning the trees, they are throwing stones at us ... this is not a demonstration anymore, this is war."
Some of the attackers were caught. Their IDs showed them to be policemen dressed in civilians clothes. Others appeared to be state sponsored "baltagiya" (gangs) and government employees. "Instead of uniformed guys trying to stop you from protesting. You've got non-uniformed guys trying to stop you from protesting," Naguib said.
AP points out:
Supporters of President Hosni Mubarak charged into Cairo's central square on horses and camels brandishing whips while others rained firebombs from rooftops in what appeared to be an orchestrated assault against protesters trying to topple Egypt's leader of 30 years. Three people died and 600 were injured.
The protesters accused Mubarak's regime of unleashing a force of paid thugs and plainclothes police to crush their unprecedented 9-day-old movement, a day after the 82-year-old president refused to step down. They showed off police ID badges they said were wrested from their attackers. Some government workers said their employers ordered them into the streets.
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"If any of the violence is instigated by the government, it should stop immediately," said White House Press Secretary Robert Gibbs.
CNN writes:
The United States doesn't know the identity of "thugs" who attacked anti-government protesters Wednesday in Egypt, but others have identified them as "supporters of the government," U.S. State Department spokesman P.J. Crowley told reporters.
"This was clearly an attempt at intimidating the protesters," Crowley said.
And Al Jazeera reports:
This is just like when the British police attacked the non-violent protesters led by Gandhi, or the police in towns in the South of the United States attacked the peaceful protesters led by Martin Luther King, Jr.Protesters in Tahrir Square shows the Al Jazeera camera the ID cards of accused plain clothed security (police ID) who came in earlier to create chaos.
Note: The police were also the ones doing most, if not all, of the looting. See this and this.