Violent clashes across the Egyptian cities of Alexandria and Port Said have left three people dead and more than 70 others injured.
Two people were killed in Alexandria. One of them was an American citizen, the US State Department confirmed.
He has been identified as Andrew Pochter, 21, from Maryland, who was a student at Kenyon College in Ohio.
He had been working as an intern at Amideast, an American non-profit organisation, a statement from the college said.
Mr Pochter died from a stab wound to the chest after violence erupted between supporters and opponents of President Mohamed Morsi.
A protester hurls a rock towards riot police in Alexandria The unrest is seen by many as a prelude to mass anti-Morsi protests planned for Sunday to mark his first year in office.
General Amin Ezzeddin, a senior Alexandria security official, said the American was using a mobile phone camera near an office of Mr Morsi's Muslim Brotherhood in the city's Sidi Gaber neighbourhood when it was being attacked by protesters.
He was rushed to a military hospital, where he died.
A second victim was shot dead during clashes in the city, while a third person died as protests also turned violent in Port Said.
The deaths come as leading clerics warned of "civil war" in Egypt after violence in the last week has left several dead and hundreds wounded.
A supporter of Mr Morsi and the Muslim Brotherhood in Cairo They backed Mr Morsi's offer to talk to opposition groups ahead of mass protests scheduled for Sunday.
State news agency MENA said 70 people had been injured.
TV footage showed protesters running from the scene as gunshots were heard.
The offices of the Freedom and Justice Party (FJP), the political arm of Mr Morsi's party, were also set on fire during the confrontations.
A Brotherhood member was also killed overnight in an attack on a party office at Zagazig, in the heavily populated Nile Delta, where much of the recent violence has been concentrated.
Anti-government protesters start a fire outside an FJP office in Alexandria Mr Morsi's movement said five supporters in all had died this week - three in Mansura and two in Zagazig.
The June 30 protest was called by Tamarod, a grassroots movement which says it has more than 22 million signatures for a petition demanding Mr Morsi's resignation and a snap election.
It alleges that Mr Morsi reneged on his promise to be a president for all Egyptians and has failed to deliver on the uprising's aspirations for freedom and social justice. Mr Morsi's supporters have questioned the authenticity of the signatures.
The president himself warned in a televised speech on Wednesday that the growing polarisation threatens to "paralyse" Egypt.
The army, which oversaw the transition from former president Hosni Mubarak's autocratic rule but has been on the sidelines since Mr Morsi's election, warned it would intervene if violence erupts.
It has brought in reinforcements to key cities, security officials said.
In an updated travel warning, the State Department cautioned US citizens "to defer non-essential travel to Egypt at this time due to the continuing possibility of political and social unrest".
President Barack Obama said during a visit to South Africa that the "most immediate concern" was to protect US embassies and consulates in Egypt.
Extreme temperatures are expected in California
A tourist protects herself from the sun on the Hollywood Walk of Fame
Mr Mandela is critically ill in hospital
Scenes outside the hospital where Mr Mandela is being treated
The tomb was filled with female mummies about 1,300 years old
The site was kept secret for fear of grave robbers
The skeletons are of Wari people, who ruled between 600 and 1100 AD
The tomb is at a coastal pyramid site
Most of the skeletons were sitting upright when they were found
The find sheds light on the position of women in Wari society
The Wari empire ruled long before the Incas
Mandela pictures are hung up at a prayer meeting at a Cape Town school
Mr Obama has described Mr Mandela as "a hero for the world"
Skies lit up as the meteor burned up over Chelyabinsk
The meteor's force was detected by nuclear test monitoring stations
The attack on the teenager happened in the Tepe district of Marmaris
Marmaris is a popular holiday resort
South Africans have gathered outside the Medi-Clinic Heart Hospital
Balloons were released outside the hospital
Crowds have been singing and dancing outside the Medi-Clinic Heart Hospital
Michelle Obama and her daughters met Mr Mandela during a trip in 2011
Sky's Mark Stone talks to Chip Starnes through a window in the factory
Workers block journalists from entering the compound in Qiaozi
The dispute is over unpaid wages and fears of factory closure
Jetskier Dale Villa
A Guardia boat like the one from which shots were fired pic: David Merrett
A car carrying Mandela family elders arrives at the ex-leader's Qunu home
Opposition leader Tony Abbott
Coy Mathis (bottom left) with family at the GLAAD Media Awards in March
General Idris: 'Frustrations are growing, we need help now'
A Syrian rebels' weapons factory
Free Syrian Army fighters launch a rocket
A Free Syrian Army fighter in the Ain Tarma neighbourhood of Damascus
Dozens congratulated the Emir (l) and his son after the abdication ceremony 
Qatar's abdicating Emir Sheikh Hamad and new Emir Sheikh Tamim
Qatar has the world's third largest gas reserves
Despite its riches, Qatar is surrounded by much bigger oil states
Qatar is the majority owner of London's The Shard skyscraper
Lionel Messi plays in a Qatar Foundation sponsored Barcelona FC shirt 
The street where the shooting took place
Mr Zuma visited Mr Mandela in hospital in Pretoria on Monday evening
Children have been sending "get well soon" messages to Mr Mandela
The hotel where Gandolfini was staying
Michael Kobold
Britain's Foreign Secretary William Hague a the meeting in Doha