Egyptian security forces fired weapons and used batons and tear gas for a fifth day in the latest security operation to clear Cairo's central Tahrir Square of opponents of army rule.
The sound of heavy gunfire rang out as police and soldiers charged hundreds of protesters at dawn on Tuesday, activists and a witness speaking to the Reuters news agency said.
"Hundreds of state security forces and the army entered the square and began firing heavily. They chased protesters and burned anything in their way, including medical supplies and blankets," protester Ismail said by telephone.
Thirteen people have been killed and hundreds wounded in clashes since Friday, while scores have also been detained in the wake of the clashes.
The latest confrontation came after Egypt's ruling military council claimed on Monday it had uncovered a plot to burn down parliament.
General Adel Emara, a member of the ruling military council, interrupted a live news conference to say he had received a call about a "plot to burn parliament and there are now large crowds in Tahrir Square ready to implement the plan".
Emara defended the military's use of force against the protesters, saying the army had a duty to protect the nation's installations.
Investigation under way
"What are we supposed to do when protesters break the law?" Emara asked. "Should we invite people from abroad to govern our nation?"
He said an investigation into the clashes and the media's coverage of them was under way. "The media is helping sabotage the state. This is certain."
Al Jazeera's Sherine Tadros, reporting from the capital, Cairo, said the news conference was one of the most defensive given by a member of the ruling military council.
"Essentially the picture that's being painted is one where the army - and by extension the country - is under attack by these counter-revolutionaries who are trying to attack soldiers and buildings ... and have no real ideology or aim behind what they are doing," she said.
The violence has drawn condemnation from Ban Ki-moon, the UN secretary-general, who "is highly alarmed by the excessive use of force ... by the security forces against protesters".
Ban called for the "transitional authorities to act with restraint and uphold human rights, including the right to peaceful protest", according to a statement from his office.
The US secretary of state also weighed in, saying she was "deeply concerned about the continuing reports of violence".
"I urge Egyptian security forces to respect and protect the universal rights of all Egyptians, including the rights to peaceful free expression and assembly," said Hillary Clinton.
The violence broke out just after the second stage of a six-week election for Egypt's new parliament that starts the slow countdown to the army's return to barracks. The military has pledged to hand power to an elected president by July.