Supporters and opponents of ousted Egyptian President Mohamed Morsi have clashed in central Cairo and north of the capital, hurling stones at each other as security forces fired tear gas to try to disperse them, witnesses said Al Jazeera reported.
Al Jazeera sources say at least four people have been killed in the violence, which erupted when the two groups came into contact on Monday evening.
One of the dead was killed on the outskirts of Tahrir Square, a hotseat for anti-Morsi demonstrators throughout the protests, while a further three were killed in Qalyoub City, sources say.
Further clashes took place later in the night at pro-Morsi rallies at Cairo University and near Salah Salem Street.
At least 16 people were injured when the protest march near Cairo University came under attack by area residents, the interior ministry said. Witnesses reported the use of birdshot, gunfire and molotov cocktails. Police then intervened, firing tear gas to disperse the crowd.
A similar attack took place on the march near Salah Salem Street.
Earlier in the day, Adly Mansour, Egypt's interim president, used the anniversary of the 1952 July 23 revolution to address the nation and re-iterate appeals for reconciliation, without outlining solid terms for how that could be achieved.
"We want to turn a new page in the country's book with no hatred, no malice, no division," he said in a pre-recorded speech that also highlighted the importance of the army in Egypt's history.
Worst violence
Al Jazeera's Rawya Rageh, reporting from Cairo, said that two people in Qalyoub had been killed by gunfire and the third was hit by a train while fleeing the violence.
The late night clashes in Cairo brought the number of injured on Monday to 43, with three said to be in critical condition, in the worst violence in the Egyptian capital since July 16, when seven died in confrontations.
Injured people had been removed from the fighting and seven Morsi supporters were detained, according to state media.
Al Jazeera's Nicole Johnston, reporting from Cairo, said that it was impossible to verify how the clash at Tahrir Square initially began.
"It certainly started off peacefully, and then the anti-Morsi crowd met the pro-Morsi crowd and the fighting began," our correspondent said.
"It has been reported that people on both sides have guns. Normally we do not see violence so early in the day and the police intervened very quickly, firing teargas in an attempt to disperse the crowd.
"At this stage that does not seem to have worked."
Al Jazeera's Rageh reported that violence had fallen into "a recurrent pattern" over the last few days of pro-Morsi protests coming under attack by people in plainclothes, rather than security forces.
The day's deadliest violence started when several hundred pro-Morsi protesters were marching towards the US Embassy to hold a demonstration against alleged American meddling in Egypt's affairs, the official said.
The march passed an entrance to Tahrir Square manned by anti-Morsi protesters, and the two sides pelted each other with rocks. Gunfire was heard, but it was not clear who was firing.
Armoured vehicles moved and blocked the road, the official said, speaking on condition of anonymity because he was not authorised to speak to the press.
The Muslim Brotherhood has staged daily protests across the country since Morsi was deposed on July 3, demanding his reinstatement.
By late on Monday, pro-Morsi protests were ongoing in at least 12 cities, including Aswan, Cairo, Alexandria, Assuit, Minya, Mansoura, El Arish, Fayoum, Marsa Matrouh, Giza, Beheira and Beni Suef.