Government forces stormed a protest camp in the southern city of Taiz, leaving at least 20 people dead Monday, according to a medical official, and marking a new level of violence in President Ali Abdullah Saleh’s attempts to stamp out the uprising against him.

Hundreds of security forces backed by armed men in plain clothes stormed the city’s Freedom Square on Sunday night, Bushra al-Maktari, a protest organizer, said by telephone. “They have attacked us with heavy gunfire, tear gas, water cannons and loud stun grenades,” she said. “They then set dozens of tents on fire and bulldozed hundreds of other tents without checking whether anyone was still inside.”

By Monday, the camp had been destroyed. More than 20 protesters had been killed and about 200 injured with live bullets, according to Sadek al-Shujaa, head of a field clinic in Taiz. Reached by phone, he said that hundreds of others were affected by tear gas inhalation and that 25 of the injuries were serious.

The U.S. Embassy in Sanaa, the Yemeni capital, condemned the violence, calling it an “unprovoked and unjustified attack on peaceful demonstrators in Taiz,” and commended the “resolve and restraint” of protesters.

The level of violence has been rising across the country since May 22, when Saleh refused to sign a deal, brokered by the Gulf Cooperation Council, that set the terms for his departure.

The crackdown in Taiz came just a day after Islamist militants appeared to take control of the southern city of Zinjibar. Yemeni warplanes on Monday carried out airstrikes against militant-held positions, Esam Mohammed, a resident, said by phone.

Saleh has repeatedly warned that al-Qaeda’s Yemeni branch would seize control of the country if he steps down. But his opponents, including defected military leaders, allege that the apparent takeover of Zinjibar is simply a ploy by the president to convince his allies that they should continue to support him.

Demonstrators in Taiz have been the driving force behind protests that have spread across the country in the past three months, posing a serious threat to Saleh’s 32 years of rule. More than 170 people have been killed in street clashes since February, according to human rights organizations.

But the crackdown in Taiz followed a Sunday pledge by Saleh to “confront a challenge with a challenge,” and it marked the first time that security forces have gone in with such force to clear out and control a protest site.

“They have attacked us in a brutal way that made us realize that this regime understands no language but force,” said Hussein al-Suhaili, a protester in Taiz.

The violence in Taiz continued into Monday evening, when police fatally shot a protester as hundreds tried to retake the square, Salah al-Dakak, a protest organizer, said.

Ahmed al-Zurkah, an independent political analyst, said security forces can be expected to use similar tactics against protesters again.

“Such violence against protesters in Taiz is likely to move to other areas where people are peaceful in order to . . . show his army is still strong,” Zurkah said.