More than 231 people have been injured in clashes between groups of armed men and pro-reform protesters marching towards Egypt's ministry of defence in the capital, Cairo, the country's health ministry said.
Thousands calling for the "downfall" of the country's ruling military council were trying to reach the military headquarters on Saturday when they were attacked by opponents armed with knives and sticks.
Witnesses said most of the injuries occurred when civilians, believed to be thugs, standing in front of military blockades hurled barrages of stones and at least six firebombs at demonstrators. The demonstrators fought back with stones torn up from the pavement.
Al Jazeera's Ayman Mohyeldin, reporting from Cairo, confirmed that "people with knives, sticks and petrol bombs surrounded the peaceful protesters" and assaulted them with their weapons.
"The situation is extremely tense, the military has used tear gas and fired into the air to push back crowds."
He also said that besides firing warning shots, the military did not intervene in the clashes.
"But they seem to have melted away from the scene. The military is nowhere to be seen compared to just moments ago they were widely present in the streets."
Ambulances were seen tending to the injured, as an army helicopter flew overhead shining its spotlight into the crowd, the AFP news agency reported.
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It is the second time protesters have tried to reach the defence ministry, after a similar attempt was quashed overnight.
The clashes came a day after military police fired shots in the air and beat demonstrators blocking a main road in the Mediterranean city of Alexandria, witnesses said.
Friday's events were a rare display of violence in two weeks of largely peaceful protests in Alexandria, Cairo and Suez following a court decision to free on bail 10 policemen accused of killing protesters during the uprising that overthrew President Hosni Mubarak in February.
Witnesses told the Reuters news agency that the clash in Alexandria erupted after hundreds of protesters blocking the coastal road near the army's northern command headquarters refused to leave the area.
More than five months after mass street demonstrations drove Mubarak from power, many Egyptians worry that their "revolution" has stalled under the military council's assumption of power.
Egypt's interim rulers have reshuffled Prime Minister Essam Sharaf's cabinet and promised to speed up trials and political reforms, but thousands kept up protests across Egypt on Friday to back demands for the policemen's trials to be held soon.
Sharaf, in a speech after his new cabinet was sworn in on Thursday, promised to set up an anti-corruption body and work to scrap a 30-year-old emergency law. He also said the interior minister would appoint a human rights adviser, and human rights and civil society groups would have access to prisons.
But activists said this was not enough.