Associated Press
In this citizen journalism image made on a mobile phone, Syrian anti-government protesters call for freedom and an end to killings.

BEIRUT — In a striking show of strength, the popular movement opposing Syrian President Bashar Assad took the streets in large numbers across the country after weekly prayers, defying a campaign of violence and mass detentions by security forces.

Protesters' exuberance in demonstrations from the Mediterranean coast to the east border, and north to south, appeared to catch authorities and even some activists off guard. Assad's aides had publicly claimed in recent days to have gained the upper hand. Some activists had tried to lower expectations for the protests after Friday prayers.

Instead, the size and scale of the civil disobedience appeared to show a new level of determination by a movement now loudly demanding an end to the 48-year-old regime run by Assad and a small group relatives and cohorts.

Robust demonstrations broke out in the capital, Damascus, including in the Muhajereen district close to Assad's residence, a sign they were spreading to the very center of power.

"We knew that there would be a high price to pay for our freedom, but we've taken the first steps now, finally, and we will not turn back," said a 50-year-old Damascus woman who took part in the protests, speaking on condition she not be named.

Protests also erupted in and around the besieged cities of Homs and Daraa, where tanks have fired at residential neighborhoods and security officials have conducted house-to-hours raids in search of protesters.

"Zenga, zenga, dar, dar, we want your head, Bashar!" protesters in Homs chanted, borrowing a highly satirized phrase used by Libyan strongman Moammar Gadhafi to emphasize his determination to fight the ongoing rebellion against his rule alley by alley and house by house.

It remained unclear whether the protests and the country's leaderless opposition could bring about the downfall of the Assad's Baath Party regime, which appears to have maintained a tight grip over security forces.

A group calling itself the National Council for the Support of the Syrian Democratic Uprising has been gathering signatures for a statement, obtained by The Los Angeles Times, demanding that security forces stop killing, arresting and besieging Syrians, avoid dragging the army into a battle with the people, end a propaganda campaign by the official media, release detainees and investigate human rights abuses. It also called for a new constitution that ends the Baath Party's monopoly on power.

By many accounts, it was a banner day for Assad's opponents. According to a trove of video footage uploaded to the Internet, protests broke out from the coastal city of Banias to the far eastern cities of Deir el-Zor; in the ethnic Kurdish cities of Qamishli and Amouda, and restive suburbs and satellite cities that ring Damascus. In the village of Kafr Nabil, one video showed onlookers tossing rose petals upon protesters marching through the city.

"With our souls, with our blood, we sacrifice ourselves for the martyrs," demonstrators chanted in the northern city of Raqqa.

The Syrian regime's repression has alarmed an international community seeking to support pro-democracy movements inspired by revolutions this year in Egypt and Tunisia. Uprisings against autocratic regimes have stalled as authorities in Syria, Bahrain and Libya resort to extreme violence and repression.

As violence has intensified in Syria, Obama administration officials have stepped up their condemnations and said privately that they may expand U.S. sanctions to include more Syrian leaders, possibly including Assad.

Jay Carney, the White House press secretary, said Friday that "absent significant changes in the Syrian government's current approach, the U.S. and its international partners will take additional steps to make clear our strong opposition to the Syrian government's treatment of its people."

Still, U.S. officials stopped short of saying that Assad had lost legitimacy, as they previously said of Gadhafi and Egyptian President Hosni Mubarak.

The extent of the protests in Syria appeared to surprise analysts and Western diplomats who'd assumed the crackdown was slowing the momentum of the two-month uprising against a regime that is a lynchpin of the volatile region's security architecture.

Activists said that at least six protesters were killed across the country. In one piece of footage said to have been filmed in Damascus, shots are heard as protesters run for cover. "They are killing someone over there," a man shouts.

Citing human rights groups, the United Nations High Commissioner for Human Rights said Friday that between 700 and 850 people had been killed in the uprising, which was sparked by the detention and torture of teenagers accused of writing political graffiti in the southern city of Daraa.

Security forces appeared to limit their use of lethal force on Friday, in what analysts and activists have speculated is a move toward tactics used by Iranian authorities to quell a 2009 uprising.

After failing to stop the movement by opening fire on unarmed demonstrators, Syrian authorities resorted to a campaign of mass arrests, imprisonments and physical abuse. Human rights activists say at least 9,500 have been detained, with some in Daraa and Banias rounded up and herded into local soccer stadiums. One piece of video footage showed a woman being stuffed into a waiting van near Damascus' Arnous Square.

A man in his 30s, who works as a cleaner at a Damascus office, said he was recently picked up by plainclothes security officers in a random arrest in his neighborhood that netted around 170 people.

He said was taken in for questioning in a makeshift detention center and forced to watch as security officials reviewed video footage of the protests in his district. He was released when they were satisfied his face did not appear in any of the footage.

"Those spotted in the clips were taken away to an unknown location," said the man, who spoke on condition of anonymity.

The regime has also started increasing the use of irregular forces, called shabiha, to squelch the rebellion."We are about to root out the rats from Homs, and from every other place in Syria," said Fadi, a self-described shabiha member in Homs who declined to give his last name.

But the Friday protests strongly suggested that authorities had little control without the presence of specialized military units, often led by Assad's relatives or members of his Alawite sect.

Amateur video footage posted to the Internet showed a group of men climbing atop office building in the western city of Hama and tearing down a huge portrait of Assad with their hands amid loud cheers and whistles in the background.

In Homs, protests broke out even in the Bab Amr district, which has been under military siege and subject to mass arrests, with the protesters' demands becoming harsher and more insulting.

"There's no more diplomacy in the slogans they are adopting," said an activist in the city reached by telephone. He described young men cursing in the face of security officials. "They're looking death in the face," he said.

For the first time since the weekly protests broke out, large numbers of women could be seen in video footage from the town of Harra, near Daraa, and in Hama.