Massive protests calling for the ouster of Syria’s authoritarian president turned deadly again Friday, with witnesses and human rights workers reporting the deaths of as many as three dozen protesters across the country and the government saying for the first time that members of its security forces had also been killed.

As the uprising demanding the resignation of President Bashar al-Assad entered its fourth week, protesters took to the streets after Friday noon prayers chanting, “God, freedom, Syria!” in the volatile southern city of Daraa; in the capital, Damascus; and in the cities of Latakia, Tartous, Homs and Harasta, according to a human rights activist in Damascus who has been in touch with other activists around Syria.

In several places, clashes broke out between demonstrators and security forces, with the worst violence occurring in Daraa, where the protests originated last month. At least 25 people were killed in the city and demonstrators had turned the al-Omari mosque into a hospital, rights activists said.

“The situation there is disastrous,” the activist in Damascus said, speaking on the condition of anonymity for fear of reprisals.

Ammar Qurabi, chairman of the Syrian National Organization for Human Rights, said at least 37 people died nationwide, including 30 in and around Daraa. Qurabi, who is in Egypt, said outages of cellphone service forced human rights workers to use service from neighboring Jordan to follow events there.

President Obama issued a strong condemnation of the violence, calling it “abhorrent.” “It is time for the Syrian government to stop repressing its citizens and to listen to the voices of the Syrian people calling for meaningful political and economic reforms,” Obama said in a statement.

State-run media reported that 19 security personnel were killed and 75 injured in Daraa. The Sana news agency blamed the shootings on “armed groups” firing from rooftops and masked gunmen riding motorcycles through the crowds. Witnesses also described the torching of a building belonging to the ruling Baath Party.

“There are families, not just young people” among the demonstrators, the activist said.

The protests appear to have unnerved the regime led by Assad, whose family has controlled Syria for more than 40 years. The 45-year-old president, who inherited power from his father 11 years ago, was initially seen as a reformer, but he has remained an autocrat whose country is one of the most rigid in the Middle East.

In Cairo, an estimated 100,000 protesters flocked to Tahrir Square after midday prayers to stage a day-long peaceful demonstration demanding that ousted president Hosni Mubarak and members of his former government be held to account for repressing and looting the country.

But early Saturday, the military sent troops and several armored cars into the square to forcibly clear the area of remaining protesters, who included about a dozen uniformed soldiers. They had joined the demonstration to urge the Supreme Council of the Armed Forces to put Mubarak on trial for corruption.

The civilian protesters, who were mostly young people, tried to form a shield around the rebellious soldiers, but they scattered after the security forces started firing into the air. The troops detained the protesting soldiers and chased demonstrators from the square.

In Yemen, witnesses and medical officials said security forces fired guns and tear gas canisters in the southern city of Taiz, killing three people and injuring scores of others, during protests calling for President Ali Abdullah Saleh to step down. But Saleh, who has been in power for 32 years, issued a defiant statement suggesting he had no plans to relinquish control or begin talks with the opposition.

Tens of thousands of pro- and anti-government demonstrators also took to the streets in the capital, Sanaa.

In Jordan, a man was in critical condition after setting himself on fire Thursday in front of the prime minister’s office, in the first such act of protest there, emulating the action of a Tunisian vendor late last year that triggered a chain reaction of protests across the region.

Correspondent Sudarsan Raghavan in Sanaa and special correspondent Haitham Tabei in Cairo contributed to this report.