Story highlights The United States calls on security forces, protesters to refrain from violence

A video of a woman's beating fuels new anger in Cairo

"Our troops do not just attack people for no reason," military spokesman says

The protests have left 11 dead and 500 wounded since Friday

Pro-democracy demonstrators battled Egyptian police for a third straight day Sunday, their anger stoked by images of a military police officer stomping on a woman's exposed stomach over the weekend.

The latest round of street clashes has left at least 10 people dead and 500 wounded since Friday, said Dr. Hisham Sheeha, a spokesman for Egypt's health ministry. An 11th person, a boy arrested Saturday, died in police custody from his wounds, the boy's attorney, Ragia Omran, said Sunday.

Cairo's stock exchange plunged amid the new turmoil, while Saturday's images of the woman's beating appeared to draw more people to the streets.

"I will go down and fight the army and retrieve the honor of this woman and those martyrs killed for the sake of Egypt's future," taxi driver Ahmed Fahmy told CNN.

The woman and a male companion were set upon by more than 20 police officers during Saturday's demonstrations in Cairo. She been dressed in a traditional robe and headscarf -- but as police clubbed her and dragged her down the street, those items were pulled away, exposing her midriff and blue brassiere in a country known for its Islamic conservatism.

Then one of the police officers aimed a foot at her upper abdomen and stamped squarely on it, while another officer jumped on the man as he lay on the pavement nearby.

"The army were like vultures who found a prey," said Mohamed Zeidan, who filmed the beating from a balcony overlooking Tahrir Square. He said after he stopped filming the beating out of fear of being discovered, "The soldiers even beat an older couple who tried to help her up."

A CNN crew that managed to escape Saturday's chaos witnessed other beatings, with children, the elderly and people on their way to work finding themselves on the end of police truncheons.

Images of the woman's treatment were splashed across the front pages of Egyptian newspapers on Sunday and zipped around the world on social media networks. But a spokesman for the military, which has ruled Egypt since February's ouster of longtime autocrat Hosni Mubarak, had no apologies.

"What was woman like her doing in a conflict zone?" asked the spokesman, Maj. Mohamed Askar. "She must have participated in the attacks on the military and the Cabinet."

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Askar questioned why the woman has not come forward to identify herself.

"Our troops do not just attack people for no reason," he said. "If she had nothing to hide then she would have presented herself. Where is she?"

Two people who know her, including the man seen being beaten alongside her in the video, told CNN the woman is a political activist and student. She does not want to speak to reporters now, but a journalist who saw the incident and was beaten as well said the woman suffered "serious bruises and cuts" as a result.

"I started to run, but she froze and fell to the ground when another protester bumped into her," said Hassan Mahmoud, the journalist. "I tried to help her get up, but the soldiers were brutal even when I told them I was a journalist. They even continued to beat her after her body was exposed."

Some Egyptians questioned the authenticity of the video. Cairo businessman Shehab Ali said there was "something fishy" about it to him.

"The army officer is wearing a pair of sneakers, which is not standard military attire considering they are all in full gear and wearing flak jackets," he said. "And how come the woman is not wearing a t-shirt or anything else under the traditional Arabic robe, although it's freezing cold and winter now?"

But cabdriver Fahmy said the images brought tears to his eyes.

"I grew up admiring our army and chanted, 'The army and the people, one hand,' " he said. "This seems like the enemy."

U.S. Secretary of State Hillary Clinton condemned the reports of violence on Sunday and said her thoughts are with the families of those killed or injured.

"I urge Egyptian security forces to respect and protect the universal rights of all Egyptians, including the rights to peaceful free expression and assembly ... Those who are protesting should do so peacefully and refrain from acts of violence," she said in a statement.

Tahrir Square has been the epicenter of anti-government demonstrations since last January, when the revolt against Mubarak began. Fresh protests sprouted in November, when the generals named Mubarak-era premier Kamal Ganzouri as a caretaker prime minister until parliamentary elections are complete.

Hundreds of police and troops swept through the plaza on Saturday, barricading nearby streets, chasing off protesters and setting tents on fire. Authorities arrested 14 people Friday and 150 on Saturday, nine of them women, said Adel Saeed, the spokesman for the general prosecutor's office.

"They have been accused of inciting violence, resisting arrest, throwing Molotov cocktails and rocks at security forces, burning public property," Saeed said.

Sunday, demonstrators hurled more rocks and Molotov cocktails and police and soldiers, injuring 58 of them, Interior Ministry spokesman Gen. Marwan Mustapha said.

Protesters captured an army officer in uniform and beat him before debating whether to swap him for a demonstrator who has been captured by the military. Several newly elected lawmakers and religious leaders from Cairo's al-Azhar University tried to negotiate a truce between protesters and security forces, but failed and were asked to leave.

The Egyptian stock market went into a nosedive amid the latest violence, losing 6 billion Egyptian pounds (about $1 billion) at its opening Sunday and finishing down nearly 3.5%.

Ganzouri criticized the unrest as an "attack on revolution" Saturday and condemned protesters for throwing stones and damaging public property. He also denied that security forces were using live ammunition against demonstrators.

In addition, 213-year-old Egyptian maps and other artifacts were destroyed after a library in Cairo was set ablaze during Saturday's clashes, officials said.

The original manuscript of the "description of Egypt" and "irreplaceable maps and historical manuscripts preserved by many generations since the building of the Scientific Center in August 1798 during the French Campaign" were destroyed in the fire, Ganzouri said in a statement.