Clashes break out at protests in northern Egypt28 June 2013 Last updated at 22:59 GMT
At least one person was killed in Alexandria as protesters stormed a local Muslim Brotherhood office
There have been deadly clashes in Egypt as both supporters and opponents of President Mohammed Morsi staged mass demonstrations across the country.
Tension has risen ahead of a protest planned by the opposition on Sunday, calling on Mr Morsi to step down.
In Cairo, thousands of Morsi supporters rallied outside the main mosque.
At least two people, one said by state TV to be a US journalist, were killed in the city of Alexandria as protesters stormed a Muslim Brotherhood office.
There are conflicting reports about how the young American, who was reported to be taking photos of clashes, died on Friday.
Egyptian officials say he was stabbed in the chest, but other reports say he was hit by gun pellets.
The US state department said it was investigating the reports of his death.
Washington also warned American nationals against all but essential travel to Egypt, and said non-emergency diplomatic staff could leave the country.
The other fatality on Friday was an Egyptian man who was shot dead, according to medical sources.
Dozens more people were injured as anti-Morsi protesters and Islamists clashed in the northern city, Egypt's second-largest.
The office of the Muslim Brotherhood, which backs Mr Morsi, was set on fire, and the authorities are reported to have called in riot police and army helicopters to try to quell the violence.
There are also reports of an explosion in Port Said, also in the north, with a local security chief saying one person was killed and five injured.
At least seven people are now believed to have died in northern Egypt in violence linked to the political situation in recent days.
Security is tight in many areas with troops deployed in Cairo and elsewhere.
Egypt's leading Muslim authority, the Al-Azhar institute, has issued a statement warning against escalating violence.
"We must be alert lest we slide into a civil war that does not differentiate between supporters and opponents," it said.
Mr Morsi's supporters are holding "open-ended" rallies before what the opposition bills as big protests on Sunday calling for him to resign. Sunday is the first anniversary of the president's inauguration.
Thousands of supporters of the Muslim Brotherhood and its Islamist allies massed outside Nasr City's Rabaa al-Adawiya mosque on Friday.
They stressed what they see as Mr Morsi's "legitimacy", rejecting the opposition's demand for him to resign.
Morsi opponents gathered in Tahrir Square and anti-Morsi protesters began a sit-in outside the building.
The main opposition coalition on Thursday rejected President Morsi's offer of dialogue.
In a statement, the National Salvation Front said it "remained determined to call for an early presidential election".
"We are confident the Egyptian people will come out in their millions to hold peaceful demonstrations on all of Egypt's squares and streets to realise their aspirations and to put the 25 January revolution back on track," it added.
The opposition was referring to the popular uprising in January 2011 which ousted President Hosni Mubarak.'Radical measures'
Mr Morsi said divisions threatened to "paralyse" Egypt, in a speech on Wednesday to mark a year in office.
Mr Morsi, who hails from the Muslim Brotherhood, became Egypt's first Islamist president on 30 June 2012, after winning an election considered free and fair.
His first year as president has been marred by constant political unrest and a sinking economy.
In his speech, President Morsi defended his performance, admitting errors and promising immediate and radical reforms to address them.
"I was right in some cases, and wrong in other cases," he said. "I have discovered after a year in charge that for the revolution to achieve its goals, it needs radical measures."
He apologised for the fuel shortages that have caused long lines at petrol stations and angered many Egyptians, and also for failing to involve the nation's youth enough.
But despite Mr Morsi's initial conciliatory tone, the speech swiftly moved into a condemnation of those he blamed for Egypt's problems, the BBC's Aleem Maqbool in Cairo reports.
"I took responsibility for a country mired in corruption and was faced with a war to make me fail," he said, naming several officials he believed wanted to "turn the clock back" to the Mubarak era, including politicians, judges and journalists.