The BBC's Quentin Sommerville: "Egypt's deep divisions were brutally exposed"
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At least 44 people have been killed and scores hurt in Egypt in clashes between police and supporters of the deposed Islamist President Mohammed Morsi.
More than 200 members of the Muslim Brotherhood were arrested in Cairo, where 40 of the deaths were reported.
Supporters of Mr Morsi marched in several cities, as the military-backed government marked the 40th anniversary of the 1973 Arab-Israeli war.
Morsi supporters say he was deposed in a military coup in July.
Hundreds of people had gathered in Cairo's Tahrir Square to celebrate the anniversary.
Jets and Apache helicopters flew overhead in formation, as part of a grand display of military hardware by the government.
The crowd cheered the flypasts, a number of people carrying portraits of defence chief Gen Abdel Fattah al-Sisi. Some want him to stand as Egypt's next president.
But the BBC's Quentin Sommerville in Cairo says supporters of Mr Morsi also took to the streets in their thousands, trying to make their way to the square and calling Gen Sisi a murderer.
Security forces used tear gas and fired into the air to stop them.
The rival protests in Cairo turned into running street battles
More than 200 members of the Muslim Brotherhood have been arrested in connection with the violence, an Egyptian security source told BBC Arabic.
Supporters of the military have been converging on Tahrir Square to mark the 40th anniversary of the 1973 Arab-Israeli war
In the upmarket Dokki district, a number of protesters were hit by live rounds, some by birdshot. In return they threw rocks at police and soldiers, our correspondent says.
The street battles raged for hours, with small fires burning and black smoke rising in several parts of the capital.
But the military succeeded in keeping the rival supporters apart, our correspondent says.
The interior ministry earlier warned it would confront any "attempts that may disturb the 6 October celebrations", the Mena state news agency reported.
In a televised address, Prime Minister Hazem Beblawi described it as a "critical time" for the country and urged Egyptians to "stand together, be optimistic about the future".
The health ministry said that in addition to the Cairo deaths, one person was killed in Delga, about 300km (190 miles) south of Cairo, and another in Bani Suef, 80km south of the capital.
There were also clashes in the Suez Canal city of Ismailiya.
Hundreds of Islamist protesters have died in violence since the Egyptian military deposed Mr Morsi in July, 13 months after he was elected as president.
He and other senior Brotherhood figures have been imprisoned and face trial.
The authorities are moving to seize the movement's assets after its activities were banned as part of a crackdown.
However, Brotherhood supporters have continued to take to the streets to protest - albeit in smaller numbers than before.