At least four people were killed in Cairo during protest marches staged by supporters of deposed President Mohamed Morsi of the Muslim Brotherhood, the Egyptian health ministry said.
Another 40 people were wounded as Morsi supporters and others against the military-backed interim government clashed with security forces in the capital and other cities across the country on Friday.
Al Jazeera's correspondent in Cairo - who cannot be named due to security reasons - said automatic weapons were fired near Tahrir Square.
"Tear gas is being fired on one of the bridges in central Cairo to prevent them from getting to Tahrir Square. In Alexandria, too, police are firing tear gas to dissuade local residents from fighting with anti-coup protesters," the correspondent reported.
Another report said that an army vehicle fired live rounds in the direction of Brotherhood supporters in the capital.
Troops blocked Tahrir Square with tanks and barbed wire as protesters chanted "down down with the murderer" - a reference to army chief General Abdel-Fatah el-Sissi, who forced Morsi out of power after millions took to the streets in the summer demanding he step down.
Meanwhile, supporters of the military clashed with Morsi supporters, firing guns and throwing rocks.
An Associated Press news agency reporter saw protesters chased by other Egyptians armed with sticks and bottles, before the two sides started hurling stones near the Egyptian museum, located at one of the main entrances of Tahrir Square.
"We will go protest and take all streets possible," said Mohammed Said, 45, during a march from the Dokki neighbourhood to Tahrir. "We will get in Tahrir at any price."
Another rally ended at a Defence Ministry building and a second at Rabaa el-Adawiyah mosque in eastern Cairo, where a pro-Morsi protest camp was violently dismantled on August 14.
Troops backed with armoured vehicles increased security around the mosque, where protesters chanted slogans against the military.
Across the country, similar clashes broke out with police firing tear gas and gunshots in the air as residents and protesters clashed and threw stones at each other.
Egypt's Muslim Brotherhood group, from which Morsi hails, is escalating protests to coincide with commemorations of Egypt's October 6 opening strike in the 1973 war with Israel.
On Friday morning, masked gunmen fired at a military vehicle near the Egyptian city of Ismailiya, killing two soldiers and wounding an officer and another soldier, a security official said.
The attack took place on the desert road between Cairo and Ismailiya when the assailants in a car without number plates opened fire, said the official.
There have been regular attacks on police and military personnel in the Suez City area, especially since the military toppled President Mohamed Morsi on July 3.
More than 100 policemen have been killed in attacks and clashes since Morsi's removal.