Cairo (CNN) -- Armed men and vendors attacked protesters in Tahrir Square on Sunday as clashes continued for hours, leaving many injured.
"The vendors always existed in Tahrir, but today around 6 p.m. they joined dozens of armed men in plainclothes who attacked us with swords, sticks and threw cooking gas canisters at our tents in the center of the square," said Kareem El Agami, a protester living in the tents since the sit-in started on June 28.
The protesters fought back with rocks and sticks as the clashes turned bloody. Ambulances sped into the square to treat dozens of people injured. Two women suffered major cuts to their scalps.
High-ranking police officers in uniform were seen at the entrance of the Egyptian museum several meters away from the Tahrir Square enclosure.
"We were moving our tents away from the center square toward the Mugama Tahrir government building when these men attacked us brutally. They were thugs sent by the police to disperse our sit-in," said Islam Ismael. His tent is now set outside the government building's garden.
Dr. Adel Aldawi, an official spokesman from the ministry of health, said he dispatched 14 ambulances to the scene. "So far 20 people were critically injured and transferred to the hospital while 27 were treated on the scene," he said.
Panic continued after the clashes ended as many witnesses who have been sleeping in the square accused the police of sparking the attack.
"We caught a low-ranking policeman in plainclothes among the attackers and checked his identification, then handed him to the military police outside the Ministry of Interior," said Dr. Yehia Abdel Shafie, a protester who was surrounded by eyewitnesses who confirmed the story.
"The policeman has been lurking around the square since we started our sit-in on June 28 and we noticed him trying to push in men between us to gather information. Today we actually saw him join the attackers and guide them toward our tents, " Ismael told CNN.
Dozens of people are continuing their sit-in at Tahrir Square in a cautious atmosphere. They are using ropes to surround their tents to keep street vendors and thugs away. Young men are directing traffic in the square and its side streets because the police are not present.
The army deployed hundreds of troops with armored vehicles onto the streets Wednesday to protect the Ministry of the Interior after the police were forced to withdraw from Tahrir Square when anti-government protests raged.
Many Egyptians are angry at the slow pace of change since President Hosni Mubarak resigned on February 11 after protests.
A group called the January 25 Coalition issued a range of demands Wednesday, one day after the latest demonstrations began.
They called for the "speedy trial of snipers and killers of protesters, the removal of Cairo's head of security and the official spokesman of the Ministry of Interior" and "an immediate investigation in the events of last night."
The group, named for the day anti-government protests began this year, also called for the release of detainees held overnight and the "immediate expulsion of security officers who continue to butcher and kill Egyptian people."
The military-led government that took over when Mubarak resigned has been prosecuting several former officials accused of ordering security forces to fire on protesters.
A police officer accused of killing 20 protesters during a January 28 demonstration has been sentenced to death. Former Interior Minister Habib El Adly has been sentenced to 12 years for corruption charges but still awaits the verdict for the charge of killing protesters.
Mubarak is scheduled to face the Cairo Criminal Court on August 3 on charges of corruption and deaths of protesters, the justice department said Wednesday.
Egypt's military rulers have set parliamentary elections for September. Protests have continued in the months since Mubarak's ouster as Egyptians have demanded speedier reforms and economic improvements.
Street vendors on mobile carts that sell tea, corn, fresh juice, sandwiches among other products have been present in Tahrir Square for months during the revolution.
The same activists that led the protests on January 25 that ousted Mubarak have called for a massive protest Friday, which has been labeled the "second revolution."
The organizers have announced that no street vendors may be present for next week's protests and national ID cards will be required for entrance to the square.