The clashes have been ongoing as Egypt's Islamist president, Mohammed Morsi, refuses to back down in a showdown over decrees granting him near-absolute powers.
The protesters don't seem to be targeting the embassy itself, Ortiz reports. Instead, they're targeting the riot police and the clashes are happening in the area and in the approaches to and from the embassy. Most embassy staff have gone home, and the embassy closed for all American citizen services.
Activists warn that Morsi's actions threaten a "second revolution," but he faces a different situation than his ousted predecessor, Hosni Mubarak: He was democratically elected and enjoys the support of the nation's most powerful political movement.
CBS News correspondent Holly Williams reported on "CBS This Morning" Thursday from Cairo that Morsi is expected to address the nation later in the day to explain why he granted himself the new powers.
Already, Morsi is rushing the work of an Islamist-dominated constitutional assembly at the heart of the power struggle, with a draft of the charter expected as early as Thursday, despite a walkout by liberal and Christian members that has raised questions about the panel's legitimacy.
The next step would be for Morsi to call a nationwide referendum on the document. If adopted, parliamentary elections would be held by the spring.
Wednesday brought a last-minute scramble to seize the momentum over Egypt's political transition. Morsi's camp announced that his Muslim Brotherhood and other Islamists will stage a massive rally in Cairo's Tahrir Square, the plaza where more than 200,000 opposition supporters gathered a day earlier.
The Islamists' choice of the square for Saturday's rally raises the possibility of clashes. Several hundred Morsi opponents are camped out there, and another group is fighting the police on a nearby street.
"It is tantamount to a declaration of war," said liberal politician Mustafa al-Naggar, speaking on the private Al-Tahrir TV station.
Morsi remains adamant that his decrees, which place him above oversight of any kind, including by the courts, are in the interest of the nation's transition to democratic rule.
Backing down may not be an option for the 60-year-old U.S.-educated engineer.
Doing so would significantly weaken him and the Brotherhood at a time when their image has been battered by widespread charges that they are too preoccupied with tightening their grip on power to effectively tackle the country's many pressing problems.
Morsi's pride is also a key factor in a country where most people look to their leader as an invincible figure.
He may not be ready to stomach another public humiliation after backing down twice since taking office in June. His attempt to reinstate parliament's Islamist-dominated lower chamber after it was disbanded in July by the Supreme Constitutional Court was overturned by that same court. Last month, Morsi was forced to reinstate the country's top prosecutor just days after firing him when the judiciary ruled it was not within his powers to do so.
Among Morsi's first acts after seizing near-absolute powers last week was to fire the prosecutor again.
Unlike last year's anti-Mubarak uprising, calls for Morsi's ouster have so far been restricted to zealous chants by protesters, with the opposition focusing its campaign on demands that he rescind his decrees, disband the constitutional panel and replace it with a more inclusive one, and fire the Cabinet of Prime Minister Hesham Kandil.
"There is no practical means for Morsi's ouster short of a coup, which is very, very unlikely," said Augustus Richard Norton, a Middle East expert from Boston University.
Still, the opposition, whose main figures played a key role in the anti-Mubarak uprising, may be tempted to try to force Morsi from office if they continue to draw massive crowds like Tuesday's rally, which rivaled some of the biggest anti-Mubarak demonstrations. They will also likely take advantage of the growing popular discontent with Morsi's government and the fragility of his mandate - he won just 51 percent of the vote in a presidential election fought against Mubarak's last prime minister.
With the country still reeling from the aftershocks of the 18-day uprising that toppled Mubarak's 29-year regime, activists and analysts warn that any escalation carries the risk of a second, and possibly bloody, revolution - pitting Islamists against non-Islamists, including liberals, women and minority Christians.
Ominous signs abound. Anti-Morsi crowds have attacked at least a dozen offices belonging to the Brotherhood across the nation since last week. Clashes between the two sides have left at least two dead and hundreds wounded.
The violence and polarization has led to warnings from some newspaper columnists and the public at large of the potential for "civil war."
"As opposed to seeking face-saving compromises, (escalation by Morsi) would indicate starkly that Egypt's leaders have increasingly come to understand the current moment in zero-sum terms," said Michael W. Hanna, an Egypt expert from the New York-based Century Foundation.
"Beyond the political dangers it poses, the move will increase the risks that the contests for power will spill over into the streets, with civil strife a real possibility."
While potentially destabilizing, Morsi's tug-of-war with the liberal opposition pales in comparison to his battle with the powerful judiciary, which considers the president's decrees an unprecedented assault on its authority.
On Wednesday, judges of the nation's highest appeals court and its lower sister court went on strike to protest the decrees, joining hundreds of other judges who have not worked since Sunday.
The Supreme Constitutional Court, which is to rule Sunday on the legality of the constitutional panel and parliament's upper chamber - both dominated by Morsi's Brotherhood and other Islamists - admonished the president for accusing it of trying to bring down his government.
The loss of the judiciary's goodwill could prove costly for Morsi.
Already, the judges are warning that, unless their demands are met, they will not assume their traditional role of supervising a referendum on a new constitution or the parliamentary elections that would follow. Without them, the legitimacy of any vote would be in question.
"This is the highest form of protest," said Nasser Amin, head of the Arab Center for the Independence of the Judiciary and the Legal Profession. "The judges felt that the constitutional declaration has taken away from them the dearest and most important mandates" - oversight of government decisions.