U.S. President Barack Obama called on Egypt's government and opposition on Saturday to engage each other in constructive dialogue and prevent violence spilling out across the region.
Bloodshed on Friday killed three people, including an American student, and mass rallies are planned for Sunday aimed at unseating Islamist President Mohamed Mursi.
Obama said he was "looking at the situation with concern".
Hundreds have been wounded and at least eight killed in street fighting for over a week as political deadlock deepens. On Friday, a bomb killed a protester at a rally by the Suez Canal. Washington is pulling non-essential staff out of Egypt.
"Every party has to denounce violence," Obama said at the other end of Africa, in Pretoria. "We'd like to see the opposition and President Mursi engage in a more constructive conversation about how they move their country forward because nobody is benefiting from the current stalemate."
He added that it was "challenging, given there is not a tradition of democracy in Egypt".
Mursi's critics hope millions will march on Sunday when he marks a year in power to demand new elections. They accuse his Muslim Brotherhood of hijacking the revolution of 2011 and using its electoral majorities to monopolize power.
"Egypt is the largest country in the Arab world," Obama said. "The entire region is concerned that, if Egypt continues with this constant instability, that has adverse effects more broadly." U.S. missions would be protected, he said. Last year, a consulate in Libya was overrun and Americans killed.
The Egyptian army, heavily funded by Washington since before Hosni Mubarak was overthrown, is on alert. It warned politicians it may step in if they lose control of the streets - an outcome some in the diffuse opposition coalition may quietly welcome, but to which Mursi's Islamist allies might respond with force.
It is unclear how big the rallies will be or when they may start. Protest organizers said on Saturday a petition calling on Mursi to quit had 22 million signatures - over 40 percent of the electorate and 7 million more than they announced 10 days ago.
The figure could not be verified, but independent analysts say there is a real prospect of very large demonstrations. Organizers have called for rallies in Cairo in the afternoon.
A few thousand activists were camping out at rival centers in the capital on Saturday. There was no sign of trouble.
Several offices of Mursi's Muslim Brotherhood were attacked on Friday, including in Alexandria where two men died, including 21-year-old American Andrew Pochter. In Port Said on the Suez Canal, a home-made hand grenade killed a protester.
The Health Ministry said 236 people were injured on Friday. The state news agency said 40 were hurt on Saturday in scuffles between rival factions in Beni Suef, south of Cairo.
The U.S. embassy evacuated non-essential staff and warned citizens to avoid Egypt. An airport source said dozens of U.S. personnel and their families left Cairo for Germany on Saturday.
The U.S. ambassador has angered liberals by saying Mursi was legitimately elected and that protests may be counter-productive for an economy crippled by unrest that has cut tourism revenues.
In the capital, Islamist supporters were still camped outside a suburban mosque where they had gathered in the many thousands on Friday to vent anger and fear over a return of army-backed rule. Some speakers also urged reconciliation.
They had their own security men, carrying staves and wearing protective gear, frisking visitors. One activist, Abdelhakim Abdelfattah, 47, said he hoped to avoid violence but that many Islamists would take to the streets if Mursi was under pressure.
"They'll come down to defend his legitimacy, not with weapons, but with their bodies," Abdelfattah said. "What's the nature of this legitimacy? The ballot box."
On Tahrir Square, seat of the uprising of early 2011, flags and tents form a base camp from where protesters plan to march to Mursi's office. Amr Riad, 26, said: "We're peaceful. But if those who come at us are violent we'll defend ourselves."
Liberal opposition leaders dismissed an offer of cooperation from Mursi this week as too little too late. The Brotherhood, which says at least five of its supporters have been killed in days of street fighting, accuses liberals of allying with those loyal to Mubarak to mount a coup against the electoral process.
The opposition says the Brotherhood are trying to hoard power, Islamize a diverse society and throttle dissent. They cite as evidence Mursi's broadsides against critical media and legal proceedings launched against journalists and satirists.
With long lines for fuel adding to economic woes, activists hope millions of the less politically engaged will protest out of disappointment that the uprising has not brought prosperity.
"Mursi is no longer the legitimate president of Egypt," Mohamed Abdelaziz, a protest organizer, told a news conference where others called for peaceful sit-ins to last until Mursi made way for an interim administration led by a senior judge.
"Come June 30, the people will run Egypt!" chanted people attending the event. The opposition, which has lost a series of elections, wants to reset the rules that emerged in a messy process of army and then Islamist rule since early 2011.
Egypt's leading religious authority warned of the risk of "civil war" after the violence of the past week. The clerics backed Mursi's offer to talk to opposition groups before Sunday's protests.
A senior figure at Cairo's Al-Azhar institute said Sunday should be a day of "community dialogue and civilized expression of opinion", a "catalyst" for political leaders to understand their national duty - and the "dangerous alternative".
The head of the Coptic Church also called for dialogue and peace. Millions of Christians worry about new Islamic laws.
Senior Brotherhood figure Essam el-Erian was dismissive of middle-class protest organizers in a Facebook post: "Millions of farmers will wake early, perform their morning prayers and go to their fields to harvest food for the people," he wrote.
Medical and security officials in Alexandria, Egypt's second city, said the American was fatally stabbed as he filmed events at the Brotherhood office in the Mediterranean port during an attack by anti-Mursi protesters, who eventually ransacked it.
Kenyon College in Ohio said Pochter was one of its students and came from Chevy Chase, Maryland. A Facebook post apparently from his family said Pochter had been teaching English to 7- and 8-year-olds and improving his Arabic. "He cared profoundly about the Middle East," the post read. "And he planned to live and work there in the pursuit of peace and understanding."