(CNN) — CAIRO, Egypt (CNN) -- Supporters of Egypt's embattled president turned out at a protest venue in the Cairo neighborhood Nasr City again Saturday. They have said they plan to demonstrate on Mohamed Morsy's behalf indefinitely.
His opponents are planning massive countrywide demonstrations Sunday, which marks a year since he took office, to call for his ouster.
Clashes between the two sides in the port city of Alexandria raged until early Saturday, state media reported.
The violence resulted in casualties.
A protester stabbed to death an American watching demonstrations Friday. Andrew Pochter, 21, of Maryland, was in the country to teach English to elementary school children. An Egyptian man also died of a gunshot wound to the head, the health ministry said.
Dozens of people were injured, state media reported.
Protesters demanding Morsy's ouster ransacked the Alexandria offices of his Freedom and Justice Party, the political wing of the religiously conservative Muslim Brotherhood.
Egypt's government deployed military security forces to patrol the streets, state media reported.
In anticipation of Sunday's rallies, Morsy met Saturday with his ministers of interior and defense to review security plans, the state-run Ahram Online news agency reported.
Nearly a week ago, Defense Minister Abdel-Fattah El-Sisi said the army would, if necessary, "prevent Egypt from slipping into a dark tunnel of civil unrest and killing, sectarianism and the collapse of state institutions," raising the specter of a return to the powerful role it played in domestic politics under President Hosni Mubarak, who was forced from power in 2011.
The violence and deployment of heavy security were reminiscent of scenes that played out then.
Accepted order vs. change
But Morsy is not Mubarak. He was democratically elected.
Those results should be respected, say his supporters, who have taken to the streets by the thousands, many of them chanting their protest slogan: "Democratic legitimacy is a red line."
Their show of support kicked off Friday in the Cairo neighborhood of Nasr City. In response, Morsy's opponents have organized nationwide large-scale rallies.
They have called for a sit-in to be held in front of the presidential palace in Cairo.
The president's opponents have spent months collecting signatures on a petition calling for Morsy to step down and call new elections.
The opposition includes a coalition of liberals, moderates and pro-democracy advocates who have joined those calling for the military to take over government and restore law and order.
Muslim Brotherhood under fire
Since Morsy took office, Egypt's already sour economy has plummeted further as investors have pulled out of the country in droves and tourism has dropped.
At the same time, crime in Egypt has shot up, and some are calling for a return to the law and order they knew under Mubarak's autocratic rule, carried out with the iron hand of the military.
Morsy gave a speech this week highlighting his achievements during his first year in office.
But opposition members said he did not address his shortcomings and called for snap presidential polls, a new government and constitutional amendments.
The Muslim Brotherhood, whose leaders included Morsy before his election, has lost four members to violence in recent days.
Two people were shot dead Thursday when armed men attacked Muslim Brotherhood offices in Zagazig, Morsy's hometown, spokesman Gihad Haddad said. The gunmen were shouting "Down with Morsy" during the assault, he said.
Amnesty International called on authorities to uphold Egyptians' right to peaceful assembly and protect protesters and bystanders from violence.
"Given the appalling track record in policing demonstrations, it is absolutely imperative that the Egyptian authorities issue very clear instructions to security forces to uphold protesters' right to freedom of assembly and refrain from unnecessary or excessive force," said Hassiba Hadj Sahraoui, Amnesty International's Middle East and North Africa deputy director.
"They should make clear that anyone responsible for arbitrary and abusive force will be brought to justice."
In the past year, about 80 people have been killed during protests and other political violence largely attributed to security forces, Amnesty said.
"We're looking at the situation with concern," President Barack Obama said Saturday in Johannesburg, South Africa. Those concerns extend to keeping U.S. embassies, consulates, diplomats and other personnel safe, he said.
"Everyone needs to denounce violence," Obama said, adding he would favor talks between Morsy and the opposition. "Nobody's benefiting now."
About 200 U.S. Marines in Sigonella, Italy, and Moron, Spain, have been put on alert as a precaution, according to two administration officials.
The U.S. State Department warned Americans to cancel all but essential travel to or within Egypt.
The United States has approved the departure of embassy and consulate employees and their dependents because of the unrest, a senior State Department official said. Nonessential employees and their relatives can get help leaving, if they choose to do so.
Pochter, the American student killed, wanted to improve his Arabic before returning to college in the United States. "He cared profoundly about the Middle East," his family said.
He was planning a career in the region in hopes of forwarding the peace process.