Images telecast by MSNBC showed protesters chasing supporters of President Hosni Mubarak away from the area where anti-government demonstrations entered their 10th day, and clashes between protesters and Mubarak supporters entered their second day. Sporadic gunfire could be heard and NBC News said pro-Mubarak elements had thrown Molotov cocktails at protesters.
There were unconfirmed reports four people were killed in the clashes, NBC said.
The U.S. State Department Wednesday advised U.S. citizens in Egypt who wish to leave on a government flight to report immediately to airport facilities.
The advisory said "further delay is not advisable" and outgoing government flights "are unlikely" are unlikely after Thursday, NBC reported.
The State Department said late Wednesday Secretary of State Hillary Clinton had called Egyptian Vice President Omar Soliman "to convey that today's violence was a shocking development after many days of consistently peaceful demonstrations."
"The Secretary urged that the Government of Egypt hold accountable those who were responsible for violent acts," the State Department said in a news release."
Clinton said the Egyptian military has exercised "restraint in the face of peaceful demonstrations" and she also urged all parties in the ongoing clashes to "recommit themselves to using only peaceful means of assembly."
The stepped up violence came one day after journalists covering the protests against Egypt's government came under attack by men identified as Mubarak supporters, who attacked protesters rallying in downtown Cairo, the Egyptian daily al-Masry al-Youm reported. At least 10 people were injured in clashes.
Witnesses said the men, some riding camels and horses, stormed the crowd and attacked protesters with swords and iron chains despite the presence of military personnel.
Journalists covering the ninth day of protests found themselves targets of violence in Tahrir Square by Mubarak supporters, The New York Times reported.
CNN correspondent Anderson Cooper reported being "set upon" by pro-government demonstrators and "punched in the head."
Reporters Without Borders said it received dozens of confirmed reports of violence against local and international journalists in Egypt. Spokeswoman Tala Dowlatshahi said the group was prepared "to expect more foreign journalists to be targeted."
"The army has failed in its commitment to protect peaceful protesters," Hassiba Hadj-Sahraoui, Amnesty International's deputy director of the Middle East and North Africa, said in a statement. "The fact that such violence is allowed to continue as they stand there begs the question whether they have orders not to interfere."
The United States also expressed its concern about the growing violence.
"After days of peaceful protests in Cairo and other cities in Egypt, today we see violent attacks on peaceful demonstrators and journalists," the U.S. State Department said in a statement. "The United States denounces these attacks and calls on all engaged in demonstrations currently taking place in Egypt to do so peacefully."
The State Department said the attacks not only were dangerous to Egypt, but also a "direct threat to the aspirations of the Egyptian people. The use of violence to intimidate the Egyptian people must stop."
At a White House press briefing Wednesday, Obama administration spokesman Robert Gibbs said "if any of the violence is being instigated by the government, it should stop immediately."
"The president and this administration strongly condemn the outrageous and deplorable violence that's taking place on the streets of Cairo -- that's taking place on the streets of Cairo today," Gibbs said.
He reiterated what President Barack Obama said Tuesday, that "the time for a transition has come and that time is now."
An international group of online hackers known as Anonymous said Wednesday it had disabled the Egyptian government's Web sites, the Times reported.
The group said about 500 supporters coordinated to use software tools to shut down Ministry of Information and National Democratic Party Web sites in support of anti-government protesters, Gregg Housh, a member of Anonymous, told the Times.
Demonstrators have called upon the 82-year-old Mubarak to end his three-decade rule now, instead of not seeking re-election in the next presidential elections in September. Obama has said "an orderly transition must be meaningful, it must be peaceful, and it must begin now."
Mubarak announced on state television Tuesday he would not seek re-election in the upcoming elections, but thousands of protesters remained in Cairo's Tahrir Square Wednesday. In Alexandria, local television reported clashes between protesters and Mubarak supporters.
Mubarak said he wanted to push through political and economic changes before stepping down.
British Prime Minister David Cameron urged the Egyptian government to present a clear timetable with details of the transfer of power, al-Masry al-Youm reported.
"President Mubarak says he is going and we respect that," Cameron said. "But what matters is not just the orderly transition but also that it is urgent, it is credible, it starts now. We should be clear we stand with those in this country who want freedom and democracy and rights the world over."
The targeting of reporters came as Internet access was restored in Egypt for the first time since last week, the Times reported.
"The Egyptian government is employing a strategy of eliminating witnesses to their actions" in a "series of deliberate attacks on journalists," said Mohamed Abdel Dayem of the Committee to Protect Journalists.
Nobel Peace Prize winner Mohamed ElBaradei, around whom opposition groups have coalesced, said allowing opposing groups to demonstrate in the same space was "calling for violence."
In an interview with the BBC, ElBaradei said the pro-Mubarak forces were "a bunch of thugs" and former members of the secret police.
If nothing else, ElBaradei said, the pro-Mubarak forces would "strengthen the resolve (of demonstrators) that Mr. Mubarak has to go.