Egypt's street protests turned violent Wednesday as factions loyal to President Hosni Mubarak clash with his opponents. Meanwhile, the U.S. government admits that it was caught off guard by the protests. The Muslim Brotherhood is denying meeting with American officials while still refusing to negotiate with the regime unless it agrees to step down.

Pro-Mubarak protesters, who may be plainclothes police officers, attacked anti-government demonstrators in Cairo's Tahrir Square. Mubarak supporters rode horses and camels into the crowd, whipping protesters while the demonstrators pulled some off their saddles and beat them. Other reports indicate the pro-Mubarak forces threw Molotov cocktails and attacked their opponents with pipes and makeshift machetes. Protesters are breaking up cement blocks on the square to make rubble to throw. The gardens outside the Cairo Museum are on fire, but the museum appears secure.

Muslim Brotherhood officials deny claims that one of their leaders met with an official at the American embassy. The group continued to call for Mubarak to resign immediately. "Be wary of Mubarak's promises of reform. What happened today is an example of the so-called reform he intends to implement," said Ashraf Abdel Ghaffar in an article on the Brotherhood's English website. "Mubarak has actively sought, and in agreement with our enemies, to destroy our nation internally and externally… Stand firm on your position and never cower to Mubarak's dictates, because with your patience, steadfastness and ability to challenge, the sun will shine and we will witness the end of our oppressors."

The U.S. government's position has not formally changed, but comments by the White House show a growing frustration with the Egyptian government. "The time for a transition has come and it has come now," said Press Secretary Robert Gibbs. He declined to elaborate. "The president of Egypt has the chance to show the world who he is" over the course of a transition from power. Gibbs also warned that any violence originating from the Egyptian government must stop "immediately."

Secretary of State Hillary Clinton told U.S. diplomats that they were in "uncharted territory."

"There are too many forces at work," she said, "some of which we are only beginning to understand. Too many cross currents and complexities."

The UN has issued statements of support for non-violent protests against the government. "I once again urge restraint to all the sides," said Secretary General Ban Ki-Moon. "An unacceptable situation is happening. Any attack against the peaceful demonstrators is unacceptable and I strongly condemn it."