Syrian security forces fired on protesters as tens of thousands of people demonstrated in several cities Friday, killing at least 20 people, wounding hundreds and forcing medical personnel to turn mosques into emergency wards, witnesses said.

Amateur video shows thousands of protesters in the southern city of Deraa. Syrian security forces fired live ammunition to disperse the demonstrators. Video courtesy of Reuters.

Friday marked the fourth consecutive week of unprecedented political turmoil challenging the rule of President Bashar al-Assad, as Syrians emboldened by a wave of pro-democracy uprisings in the Middle East have taken to the streets calling for more meaningful reform.

The southern city of Daraa, where the first protests emerged a month ago, saw the worst violence on Friday. Some reports put the number of casualties at 16 dead and hundreds injured. Witnesses said that doctors were among the dead, and said that security forces had opened fire on ambulances to prevent them from accessing the injured on the streets.

A nurse at the hospital said they had run out of beds, and that many people were being treated on the floor or in nearby mosques.

Mr. Assad has made a series of overtures in the past few weeks to contain the unrest, with little impact on the protests. He has appointed a new prime minister, released some political prisoners, reached out to Kurds by granting them citizenship and to Islamists by closing casinos and lifting a ban on veiled women in educational establishments.

The opposition says these steps are welcome, but not enough. Crowds in the Damascus suburb of Harsta shouted, “We want freedom,” and “Syrian people won’t be humiliated,” according to a video posted on the Facebook page of Syria Revolution 2011. The Facebook group has called for demonstrations every Friday to pressure Mr. Assad into more concessions, an activist working with the group said in an interview.

“We want freedom of expression, multiple political parties and justice for the criminals who killed people at the demonstrations,” said one of the activists with the Facebook page.

A witness in Harasta said security forces had clamped down on a peaceful protest, beating protesters and firing shots to prevent the crowd from growing bigger.

Another protest in the northwestern city of Banias started with demonstrators holding olive branches and calling for freedom, but calls rose for Mr. Assad’s regime to be toppled after news of the deaths in Deraa emerged, activists said.

Even Kurdish communities turned out on Friday in some northeastern towns, a day after Mr. Assad pledged to grant citizenship to 200,000 Kurds, fulfilling a decades-old demand of the country’s long-ostracized minority. Kurdish residents reached by phone said this gesture alone wasn’t enough, and they wished to see comprehensive reforms with more rights for minorities.

President Barack Obama, in a statement, said “I strongly condemn the abhorrent violence committed against peaceful protesters by the Syrian government.”

Write to Farnaz Fassihi at farnaz.fassihi@wsj.com