A US envoy met Egyptian officials Saturday amid efforts to find a peaceful solution to the stand-off between supporters of ousted Islamist president Mohamed Morsi and the army-installed interim government.
William Burns met Foreign Minister Nabil Fahmy, the official MENA news agency reported, hours after seeing members of Morsi's Muslim Brotherhood and its political arm, the Freedom and Justice Party.
The talks come as tensions mounted over the looming break-up of two sit-ins in Cairo staged by Morsi loyalists which have paralysed parts of the city and deepened divisions.
Burns' visit is his second since the army's July 3 ouster of Morsi, and comes after EU foreign policy chief Catherine Ashton visited to help broker a peaceful solution.
EU Middle East envoy Bernardino Leon and German Foreign Minister Guido Westerwelle also visited Cairo this week to urge both sides to reach a compromise.
The flurry of diplomatic activity comes as Morsi loyalists vowed to keep fighting for his reinstatement despite police calls to lift the sit-ins.
They staged defiant rallies on Friday, but plans for four later marches, including to two key army headquarters, fizzled out after police dispersed a new sit-in outside the Media Production City in a Cairo suburb.
Meanwhile, Ayman al-Zawahiri, the Egypt-born head of Al-Qaeda, accused the United States of "plotting" Morsi's overthrow with Egypt's military and its Christian minority.
"Crusaders and secularists and the Americanised army have converged ... with Gulf money and American plotting to topple Mohamed Morsi's government," he said in a 15-minute audio recording posted on militant Islamist forums.
In his first comments since the July 3 coup, Zawahiri also attacked Morsi's secular opposition and Coptic Christians, who he said wanted a secessionist state in Egypt, and called for a mass movement to instal Islamic law.
Morsi supporters began to march after Friday prayers, pouring out of several mosques in Cairo.
An early evening protest outside Cairo's media production city descended into mayhem, with at least one protester wounded by birdshot.
Police fired tear gas at protesters who had set up tents and brick fortifications outside the compound. The protesters responded with stones.
"I am a Muslim, not a terrorist," they chanted.
The interior ministry accused protesters of firing birdshot, wounding a conscript, and said police made 31 arrests.
Witnesses also reported clashes between residents in the Alf Maskan area and Morsi loyalists after they tried to establish a protest site.
Morsi supporters had announced Friday evening marches to several security facilities, including the Republican Guard headquarters where more than 50 demonstrators were killed last month.
While large crowds turned out to one march at the military intelligence headquarters, stopping short of the building and turning back after a brief protest, attempts to march to the Republican Guards appeared to have been called off.
Morsi's supporters have remained defiant in the face of mounting threats from the interim government.
The interior ministry has urged people at protest sites in Rabaa al-Adawiya and Nahda squares "to let reason and the national interest prevail, and to quickly leave".
State-owned Al-Ahram newspaper reported Friday that police had a plan to disperse the sit-ins but were holding out for a peaceful resolution.
Vice President Mohamed ElBaradei called for a halt to violence in an interview with the Washington Post.
"Once we do that, we immediately have to go into a dialogue to ensure that the Brotherhood understand that Mr Morsi failed. But that doesn't mean that the Brotherhood should be excluded in any way."
More than 250 people have been killed since Morsi's ouster.
His supporters have been angered by comments from US Secretary of State John Kerry, who told Pakistani television that Egypt's military was "restoring democracy" when it ousted him.
"Is it the job of the army to restore democracy?" asked Muslim Brotherhood spokesman Gehad al-Haddad on Friday.
Morsi has been formally remanded in custody on suspicion of offences committed when he broke out of prison during the 2011 revolt that toppled former president Hosni Mubarak.
Prosecutors have also referred three top Muslim Brotherhood leaders, including supreme guide Mohamed Badie, for prosecution on allegations of inciting the deaths of demonstrators.
Morsi was detained hours after the coup and is being held at an undisclosed location.
His family has been unable to see him, but Ashton met Morsi on Tuesday and said he was "well".