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 an American non-profit organisation teach where he was teching English to children and improving his Arabic, his family said in a statement.
"He had studied in the region, loved the culture, and planned to live and work there in the pursuit of peace and understanding," read the statement, that asked for privacy in a time of grieving.
Mr Pochter died from a stab wound to the chest after violence erupted between supporters and opponents of President Mohamed Morsi.
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.
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.
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.