As Yemen's increasingly broad protest movement sought to expand their presence in the capital tonight, the protests turned violent for the first time in weeks – underscoring the volatility of the Arab world's poorest country.

At least 10 were injured by gunfire in front of Sanaa University, according to medical workers on the scene, and dozens more were injured in clashes with security forces as the protesters sought to make space for more tents near one of the security lines. The gunfire came from both uniformed security forces and individuals in civilian clothes, witnesses said.

“I was sitting with other demonstrators when they started firing shots at the crowd," said Saleh Al Hashmy, who was bandaged and bleeding from the head outside of a nearby mosque that had been turned into a makeshift hospital. "We all got up to run and the police beat us with sticks and fired tear gas."

Another eyewitness, Mohammed Ali Hamouda, also said he had seen police fire on the protesters. Among the injured at the mosque was a man who had been shot in the eye.

According to a government statement, police attempted to apprehend armed individuals at a security checkpoint in front of the demonstration area. When they resisted arrest, a gunfight broke out, according to the statement.

The skirmishes marked the first violence since Feb. 24, when President Ali Abdullah Saleh ordered police to protect protesters. But tensions have been mounting in recent days, as Mr. Saleh rejected an offer from the opposition to come up with an exit strategy – but not actually step down – by year's end.

"The peaceful and smooth transition of power is not carried out through chaos but through the will of the people expressed through elections,” said a statement from the president’s office.

Even before tonight's clashes, Saleh's insistence on finishing his term, which expires in 2013, had already raised concern about more potential violence.

On Sunday, the US embassy urged all American citizens to leave the country, citing high security threats resulting from both terrorism and civil unrest.

At least 27 have been killed since Feb. 16, though the vast majority of the deaths have come from Aden, according to Amnesty International. But now, the capital is feeling the shocks of violence.

“It’s been peaceful here," said opposition demonstrator Shaif al-Jabry after tonight's skirmishes. "But there’s violence all over the country and now it’s coming to the capital."