Police fired tear gas in central Cairo on Monday when protesters calling for the reinstatement of the ousted Islamist president, Mohamed Mursi, scuffled with drivers and passers-by annoyed that they had blocked major roads.
Supporters of Mursi, Egypt's first freely elected president, threw rocks at police near Ramses Street, one of the capital's main thoroughfares, and on the Sixth of October Bridge over the Nile in the first outbreak of violence in Egypt in a week.
"It's the army against the people, these are our soldiers, we have no weapons," said Alaa el-Din, a 34-year-old computer engineer, clutching a laptop.
"The army is killing our brothers, you are meant to defend me and you are attacking me. The army turned against the Egyptian people."
While smaller in scale and more localized than previous clashes since Mursi was deposed by the military on July 3, scenes of running street battles will raise further concerns over stability in the Arab world's most populous country.
Eye witnesses said thousands of pro-Mursi demonstrators were in the area and police had used tear gas several times to try to control the crowd. A large fire was burning on the bridge, although the cause was not immediately clear.
The clashes came as the first senior U.S. official to visit Egypt since Mursi was toppled was snubbed by both Islamists and their opponents.
Large crowds mobilized by Mursi's Muslim Brotherhood movement gathered at various points in the city, including outside the Rabaa Adawiya mosque where they have held a three-week vigil, and at Cairo University.
The army warned demonstrators on Monday that it would respond with "the utmost severity and firmness and force" if they approached military bases.
At least 92 people were killed in the days after Mursi was toppled, more than half of them shot by troops outside a barracks near the mosque a week ago.
Protests since then had been tense but peaceful until Monday evening's developments.
U.S. ENVOY SHUNNED
The crisis in Egypt, which has a peace treaty with Israel and controls the strategic Suez Canal, has alarmed allies in the region and the West.
After meeting interim head of state Adli Mansour and Prime Minister Hazem el-Beblawi, U.S. Deputy Secretary of State William Burns insisted he was not in town "to lecture anyone".
He arrived in a divided capital where both sides are furious at the United States, which supports Egypt with $1.5 billion a year in mostly military aid.
"Only Egyptians can determine their future. I did not come with American solutions. Nor did I come to lecture anyone," Burns told a brief news conference. "We will not try to impose our model on Egypt."
Washington, never comfortable with the rise of Mursi's Muslim Brotherhood, has so far refused to say whether it views Mursi's removal as a coup, which would require it to halt aid.
The State Department said Burns would meet "civil society groups" as well as government officials. But the Islamist Nour Party and the Tamarud anti-Mursi protest movement both said they had turned down invitations to meet him.
"First, they (the Americans) need to acknowledge the new system," Tamarud founder Mahmoud Badr said. "Secondly, they must apologize for their support for the Muslim Brotherhood's party and terrorism. Then we can think about it," he told Reuters.
In a further slight, Badr posted a copy of his invitation, including the U.S. embassy's telephone number, on the Internet.
Nour, sometime allies of Mursi's Brotherhood who have accepted the army takeover, said they had rejected meeting Burns because of "unjustified" U.S. meddling in Egypt's affairs.
The Brotherhood said it had no meeting planned with Burns, although it did not make clear if it had been invited.
"America are the ones who carried out the military coup," Farid Ismail, a senior official in the Brotherhood's political arm, told Reuters. "We do not kneel for anyone, and we do not respond to pressure from anyone."
If Burns had driven through the city centre a few miles away, he might have seen a giant banner with a portrait of U.S. ambassador Anne Patterson and the message "Go home, witch!" hung by Mursi's opponents.
Mursi is being held incommunicado at an undisclosed location. He has not been charged with a crime but the authorities say they are investigating him over complaints of inciting violence, spying and wrecking the economy. Scores of Mursi supporters were rounded up after violence last week.
Many of the top Brotherhood figures have been charged with inciting violence but have not been arrested and are still at large. The public prosecutors' office announced new charges against seven Brotherhood and Islamist leaders on Monday.
Beblawi has been naming ministers for his interim cabinet, including a former ambassador to the United States as foreign minister, a sign of the importance Cairo places in its relationship with its superpower sponsor.
U.S.-educated economist Ahmed Galal, as finance minister, has the task of rescuing an economy and state finances wrecked by two and a half years of turmoil.
That task became easier, at least in the short term, after Saudi Arabia, the United Arab Emirates and Kuwait - rich Gulf Arab states happy at the downfall of the Brotherhood - promised a total of $12 billion in cash, loans and fuel.
The new planning minister, Ashraf al-Arabi, said the Arab money would be enough to sustain Egypt through its transition period and it did not need to restart talks with the International Monetary Fund.
Egypt had sought $4.8 billion in IMF aid last year, but months of talks ran aground with the government unable to agree cuts in unaffordable subsidies for food and fuel. Arabi's comments could worry investors who want the IMF to spur reform.