Egyptian Health Ministry officials say 34 people have been killed in violence outside a military building in Cairo where supporters of deposed President Mohamed Morsi were protesting his ouster.
Morsi's Muslim Brotherhood blamed the military, saying soldiers fired on the protesters Monday at the Republican Guard headquarters.
The group called for an "uprising" against what it says are those "who want to steal the revolution with tanks."
The military gave a different version of the events, saying Muslim Brotherhood members tried to storm the building, and soldiers opened fire in defense. They say one soldier was killed.
In response to Monday's violence, the ultra-conservative Salafi Nour Party says it is withdrawing from talks to form a new government.
VOA correspondent Sharon Behn, who is in Cairo, says it is hard to independently verify what happened Monday, but that the Nour Party withdrawal could further stress tensions among Egyptians.
“What people here are saying is that that could further polarize the society and divide it between what is turning out to be a sort of liberal, secular, pro-army group that wanted the ouster of Mohamed Morsi because he was heading down a path of Islamic rule that they didn’t want, and on the other side we have the more Islamic sector of the population, which thought that Mohamed Morsi was doing the right thing for the country,” she said.
The two sides have staged massive rallies in Cairo and other cities since the military ousted Morsi last Wednesday. Clashes Friday left at least 36 people dead and more than 1,000 injured nationwide.
Even before the Nour Party pullout, the country's interim leaders struggled to put together a new government acceptable to both backers and opponents of Morsi.
The political standoff continued Sunday between the secular and liberal-dominated transitional government and hardline Islamist lawmakers. The faceoff erupted Saturday, after interim President Adly Mansour's office - under pressure from Islamists - backtracked on a decision to appoint Nobel laureate and diplomat Mohamed ElBaradei as Egypt’s interim prime minister.
Many Islamists view the secular former chief of the U.N. nuclear agency an unacceptable choice because of his liberal views.
ElBaradei condemned Monday's violence and called for an independent investigation.
The transitional prime minister will have sweeping powers to govern, while the president is expected to be a largely symbolic post.
In Washington, President Barack Obama voiced renewed concern about the political upheaval, while reiterating that the United States is not aligned with and does not support any particular Egyptian political party or group.
The army described Morsi's removal as necessary to enforce the will of millions of people who have repeatedly demanded his resignation.
Watch a related report by VOA's Sharon Behn