WSJ’s Farnaz Fassihi reports a crowd of 50,000 protesters was fired upon by Syrian troops as they marched to Danaa. Also, Sam Dagher has the latest from Libya as NATO takes over No-Fly Zone patrols.

Thousands of protesters demanding political liberalization marched in cities across Syria on Friday, an unprecedented display of public dissent that prompted violent clashes with security forces and left dozens dead and injured, according to witnesses and media reports.

The protests, once unthinkable against a regime believed to have an unshakable grip on security, came a day after President Bashar al-Assad’s government announced economic and political concessions aimed at appeasing protesters and getting them off the streets.

Protests spread across Syria challenging the Syrian President Bashar al-Assad’s 11 year rule. Video courtesy of Reuters.

Some U.S., European and Israeli officials saw the potential weakening of Mr. Assad’s government—a close ally of Iran and key player in regional politics— as an important opening to significantly undermine Tehran’s role in the region.

But there remained concerns that the turmoil could usher in more wide-scale bloodshed or, should Mr. Assad fall, another regime hostile to the West.

Syria had seen little unrest before last week, amid months of protests sweeping the Middle East. That has changed dramatically in the past seven days as the arrests of some youths in one southern Syrian town spiraled into violent confrontations in that area and sparked protests across several other major cities, including the capital, Damascus.

Associated PressThousands of Syrians took to the streets Friday demanding reforms and mourning dozens of protesters who were killed during a violent crackdown.

A prolonged period of unrest in Syria could have major regional repercussions. Syria is a key conduit for Iranian influence in the Middle East, and the country wields strong influence on neighboring Lebanon and the Palestinian territories, where it supports the militant groups Hezbollah and Hamas. Syria also remains officially at war with Israel over the disputed Golan Heights.

“If Syria goes, this would be a serious setback for Iran,” said a senior European official working on the Middle East. “We would greet this with great pleasure.”

The Obama administration publicly called for Mr. Assad to end the crackdown. The White House also convened a special meeting to assess regional implications, said U.S. officials.

Senior U.S. officials were cautious Friday in predicting the outcome for Mr. Assad. But they stressed that they believed Mr. Assad’s image as a political reformer receptive to Syria’s next generation had taken a crippling blow. They noted that Mr. Assad has twice before pledged to implement the same sets of reforms that his government unveiled Thursday, but never delivered. They also said many Syrians would equate the violence in Daraa with Hafez al-Assad’s 1982 crackdown in Hama, which is estimated to have killed between 10,000 to 40,000 Syrians.

Thousands of Syrians took to the streets across the country in an unprecedented display of dissent, clashing with security forces and government supporters in several cities including Damascus, above. Dozens of people have been killed in the unrest, according to witnesses.

“I don’t know if the regime will fall. It’s not generating the mass numbers yet,” said a senior U.S. official. “But the way Bashar’s presented himself to the world, as a reformer, is crumbling.”

Syria has at times been a stable fixture in a Middle East rocked by wars and violent movements. The country has been sanctioned by Congress for supporting Hamas and Hezbollah, both of which the U.S. considers terrorist organizations. Yet the Syrian regime, especially after the Sept. 11, 2001, terrorist attacks, has intermittently provided intelligence to the U.S. in efforts to fight Islamic extremism. And Syria’s neighbors, especially Israel, worry about what might happen if the regime fell, removing a force that has held together the country’s disparate array of ethnic and sectarian groups.

Even as he aided some of Israel’s most implacable enemies, Hamas and Hezbollah, Mr. Assad kept the Golan Heights border quiet. “We will not mourn the demise of Assad, but no one can guarantee that the alternative will be any better,” said an Israeli official.

Mr. Assad came to power in 2000, succeeding his father, Hafez al-Assad, who had led the country for three decades.

A Western-trained ophthalmologist, the younger Mr. Assad entered office promising liberalization. But his regime has continued his father’s intolerance of dissent and demonstrated willingness to jail opponents or use overwhelming force against them.

The Obama administration has increasingly questioned the belief among some regional analysts that Mr. Assad has been a stabilizing force. U.S. officials argue that Syria’s alleged pursuit of a nuclear-weapons capability—a charge Damascus denies— is deeply destabilizing for the region. And they say Hezbollah and Iran seem to be dominating in the relationship with Mr. Assad.

“Assad has the iron fist and velvet glove treatment. Today has been very bloody so this strategy doesn’t seem to be working,” said Andrew Tabler, a fellow at Washington Institute for Near East Policy.

In Qatar, influential Sunni cleric Sheikh Yusuf al-Qardawi, said of the unrest, in a sermon that was broadcast on al-Jazeera and quickly posted on YouTube: “Today the train of revolution has arrived at a station that it was destined to reach, the Syrian station.” “It isn’t possible for Syria to detach itself from the history of the Arab nation.”

The images of men and women marching after Friday prayers, chanting “Free Syria,” caught many regional analysts and officials by surprise.

Across the region, from Iran to Israel, Syria’s upheavals were followed closely with varying degrees of apprehension.

Neighbors Lebanon and Jordan fear instability could spread across their borders. Jordan saw antigovernment protests flare into violence Friday.

Iran, which has been vocal in its support for uprisings in Tunisia, Egypt and Bahrain, remained silent on Syria Friday—a sign of concern, some U.S. officials said.

In the U.S., some conservative lawmakers pressed President Barack Obama to take more aggressive actions to topple the Syrian leader. “The Syrian people must know that the U.S. stands with them against the brutal Assad regime,” Sens. Jon Kyl (R., Ariz.) and Mark Kirk (R., Ill.) said in a statement. “We can ill-afford another timid embrace of a democratic uprising.”

Asked why the U.S. is taking military action to support an uprising in Libya, but not in Syria, State Department spokesman Mark Toner said “what’s happened and is happening in Libya stands apart.” He urged the Syrian government to allow its people to peacefully demonstrate for their “universal rights.”

U.S. officials, in gauging Mr. Assad’s prospects for survival, said they are focused on whether the protests become larger in major cities and whether they fuel divisions between the president’s supporters in his Alawite minority group, and other ethnic and sectarian groups.

Friday’s protests began after afternoon prayers, which have in the past few months become weekly occasions for people across the Middle East to vent anger against autocratic governments.

The largest Syrian protest took place in the southern city of Daraa, where the unrest was sparked a week ago when some youths were arrested for spray painting antigovernment slogans on a wall. Their arrest prompted massive outrage, followed by heavy government crackdowns that human-rights organizations estimate killed at least 55 people.

On Friday, tens of thousands of people took to the streets, demanding an end to the Assad government’s rule. Demonstrators set fire to a statue of Mr. Assad’s father, according to media reports, while some, in the city of Homs, tore down and kicked poster of the elder Mr. Assad, a YouTube video showed.

In the village of Sanamein, near Daraa, activists told the Associated Press that police fired into a crowd and as many as 20 people were killed and dozens injured. The reports couldn’t be independently verified.

A YouTube video showed graphic images of dead bodies said to be in Sanamein, and a man crying. Another one consoled him by saying, “They are all martyrs.”

YouTube videos showed what appeared to be a more-festive crowd in Duma preparing for what activists expected to be a continuous demonstration there. “We all want freedom, Muslims and Christians,” they chanted.

Across Syria, security forces battled the crowds with batons and sticks, in some instances firing at the public and arresting dozens of people, according to media reports and witness accounts.

In Damascus, where activists said it was much harder to act, several protests of a few hundred people erupted around mosques. Intelligence agents have been cracking down in recent days, detaining scores of people and demanding they hand over passwords to email and Facebook accounts.

Amateur video shows thousands marching in funerals for those killed by Syrian security forces.Video Courtesy of Reuters.

Hours after reports emerged of three people being shot dead in a Damascus suburb, a vast pro-Assad rally took place in the capital. Hundreds of cars plastered with pictures of the president and crammed full of youths waving the Syrian flag descended on the central Umayyad Square. Fireworks were let off into the sky. In other areas, the streets were unusually quiet.

In an attempt to counter news coverage of antigovernment demonstrations, huge rallies of pro-Assad supporters drove around the capital waving flags and photos of the president.

The Facebook page for organizing Friday’s rally “Syria Revolution 2011,” posted that more than 200,000 protestors in Daraa and Houran had one demand: “Free Syria” and “No to the presence of corrupt murderers in the government.”

—Nada Raad and Josh Mitnick contributed to this article.Write to Farnaz Fassihi at farnaz.fassihi@wsj.com and Jay Solomon at jay.solomon@wsj.com