Muslim Brotherhood protests plunged into violence across Egypt today, with around 50 killed in Cairo alone on a "Day of Rage".
Islamist followers of ousted president Mohammed Mursi called the protest to denounce Wednesday's police crackdown in which hundreds of people died.
The army deployed dozens of armoured vehicles on major roads in the capital as a crowd of several thousand marched through downtown this afternoon.
Anger on the streets was directed at army commander General Abdel Fattah al-Sisi, who moved against Mr Mursi last month after massive street rallies against his administration.
The Interior Ministry had warned that police would use live ammunition against anyone threatening public buildings.
Tear gas was fired and shooting was heard where pro-Mursi supporters were protesting.
Two witnesses said they saw protesters throw petrol bombs at a police station near the square.
A Reuters photographer said security forces opened fire from numerous directions when the station was attacked.
More than 40 people were also killed in provincial cities, taking the overall toll close to 100.
A security official said 24 policemen had been killed and 15 police stations attacked since late yesterday.
A police conscript was killed in a drive-by shooting in the north of the capital, state news agency MENA reported.
Nile TV showed footage of a gunman among Islamist protesters firing from a central Cairo bridge.
Showing no sign of wanting to back down, the Brotherhood called for a week of daily marches across the country.
A national state of emergency remains in place, as well as a dusk to dawn curfew.
Brotherhood vows to not retreat
The Muslim Brotherhood called for today's nationwide march of millions to show anger at Wednesday’s security crackdown.
At least 623 people died and thousands were wounded when police cleared out two protest camps in Cairo in protest at the overthrow of Mr Mursi.
The Brotherhood said it would not retreat in its showdown with General Sisi.
"After the blows and arrests and killings that we are facing, emotions are too high to be guided by anyone," said Brotherhood spokesman Gehad El-Haddad.
The Brotherhood accuses the military of staging a coup when it ousted Mr Mursi.
However, liberal and youth activists who backed the military saw the move as a positive response to public demands.
But some fear Egypt is turning back into the kind of police state that kept the disgraced Hosni Mubarak in power for 30 years before his removal in 2011.
In calling for a "Day of Rage," the Brotherhood used the same name as that given to the most violent day of the uprising against Mubarak.
That day, 28 January 2011, marked the protesters' victory over the police, who were forced to retreat.
In a counter move, a loose liberal and leftist coalition, the National Salvation Front, called on Egyptians to protest today against what it said was "obvious terrorism actions" conducted by the Brotherhood.
Western governments urged restraint and Germany cautioned the new government that it was reviewing its ties.
By contrast, Saudi Arabia's King Abdullah said his country stood with Egypt in its battle against "terrorism".
Travel warnings issued over situation in Egypt
The Department of Foreign Affairs has advised Irish citizens not to travel to Egypt at this time.
However, it said it is not currently advising Irish tourists at the Red Sea resort to depart.
The Embassy in Cairo remains open and is monitoring the situation.
Irish people are also urged to comply with curfew conditions and to monitor the media for updates on the security situation.
Irish people already in Egypt should "exercise extreme caution" and "avoid all protests and demonstrations".
Germany has extended its travel alert for Egypt to include the Red Sea tourist resorts, which had previously been excluded from its travel alerts, though increased caution had been recommended.
"We advise against travel to Egypt and advise urgently against travel to Cairo, upper Egypt and the Nile delta," the spokesman said.
"Foreign Minister Guido Westerwelle urges German citizens to take this travel advice very seriously."
The alert is short of a full travel warning, however, which would entail tourists being evacuated.
Meanwhile, the British Foreign Office has advised its citizens, currently on holidays in the Red Sea resort of Hurghada, to stay within the grounds of their hotels.
There were violent clashes in the area this week, although the tourist centres were not affected.