BEIRUT (AP) -- Tens of thousands of Syrian protesters shouted for President Bashar Assad's death Friday in a dramatic escalation of their rage and frustration, defying bullets and rooftop snipers after more than a week of intensified military assaults on rebellious cities, activists and witnesses said.

Security forces killed at least 11 protesters, according to human rights groups.

The calls for Assad's execution were a stark sign of how much the protest movement has changed since it erupted in March seeking minor reforms but making no calls for regime change. The protests grew dramatically over the five months that followed, driven in part by anger over the government's bloody crackdown in which rights groups say at least 1,700 civilians have been killed.

But with the regime shrugging off even the most blistering condemnation, the uprising has become a test of endurance as both sides draw on a deep well of energy and conviction. U.S. Secretary of State Hillary Rodham Clinton on Friday urged countries to stop buying Syrian oil and gas or selling the regime weapons, saying those who still do so must "get on the right side of history."

In cities around Syria, protesters chanted, "The people want to execute the president!" during the now-familiar cycle of weekly demonstrations followed by a swift crackdown by the military, security forces and pro-government gunmen who operate on the regime's behalf.

Security forces broke up protests quickly around the capital Damascus, in the central city of Homs and elsewhere, firing bullets and tear gas. Some areas saw only limited demonstrations because soldiers deployed heavily in restive areas.

In a significant show of defiance, some of the largest protests Friday were on the outskirts of the central city of Hama and in the eastern city of Deir el-Zour, where government forces seized control in major military offensives during the past week. The fact that protesters still turned out was a signal that Assad's forces cannot terrify protesters into staying home.

However, within Hama, protesters struggled to turn out in great numbers after soldiers clamped down heavily in the streets, witnesses said. Snipers were stationed on rooftops, and troops surrounded mosques and set up checkpoints to head off any marches.

"There are security checkpoints every 200 meters (655 feet), they have lists and they're searching people ... the mosques are surrounded by soldiers," a Hama-based activist told The Associated Press by telephone, speaking on condition of anonymity for fear of reprisals.

Dozens of soldiers deployed in Hama's Assi Square, which had been the main converging point for hundreds of thousands of protesters in previous weeks, the activist said.

In the central city of Homs, more than a 1,000 soldiers, security agents and plainclothes policemen were deployed in the city's main square.

At least 11 protesters were killed across the country: Five outside the capital, Damascus; one each in Homs and Hama; two in the major northern city Aleppo; one in Deir el-Zour; and one in eastern Idlib province, according to multiple activist groups. Military raids earlier in the day killed at least two people.

"Where are the prisoners, Bashar? Free the prisoners, Bashar!" shouted protesters in the Mediterranean coastal city of Latakia, shown in amateur video posted by activists. Another video showed a crowd outside a mosque in the southern city of Daraa hit by clouds of tear gas after they chanted for the downfall of the regime.

The Associated Press could not verify the videos. Syria has banned most foreign media and restricted local coverage, making it impossible to get independent confirmation of the events on the ground.

The government has justified its crackdown by saying it was dealing with terrorist gangs and criminals who were fomenting unrest.

The military offensive reflects Assad's determination to crush the uprising against his rule despite mounting international condemnation, including U.S. and European sanctions.

A flurry of foreign diplomats have rolled through Damascus urging Assad to end a campaign of killing that rights groups say has killed more than 1,700 civilians and several hundred members of the security forces since mid-March.

"We believe that President Assad's opportunity to lead the transition has passed," Jay Carney, spokesman for President Barak Obama, told reporters traveling on Air Force One on Thursday.

But the U.S. and other nations have little power to threaten further isolation or economic punishment of Assad's pro-Iranian regime - unlike in Egypt, where Obama was able to help usher longtime ally Hosni Mubarak out of power.

On Friday, the Dutch Foreign Ministry said the European Union may decide in the next week or two to broaden its sanctions against the Syrian regime and state-run businesses.

Foreign Minister Uri Rosenthal has been lobbying his colleagues to expand the EU travel ban on Syrian officials - which now covers 35 people, including Assad - and to target Syria's telecommunications, banking and energy sectors. Syria gets about 28 percent of its revenue from the oil trade.

"We need to cut off the oxygen from the regime through its profitable public enterprises," Rosenthal said on the ministry's Web site.

But the bloody crackdown has continued, along with a nationwide campaign of arrests.

Security forces on Thursday detained Abdul-Karim Rihawi, the Damascus-based head of the Syrian Human Rights League, activists said. A longtime rights activist, Rihawi had been tracking government violations and documenting deaths in Syria.

He was picked up from a cafe in central Damascus along with a journalist who had been interviewing him, according to rights activist Ammar Qurabi.

Italy and France on Friday condemned the arrest and called for his immediate release.

"By its brutal and symbolic character, the arrest of Abdul-Karim Rihawi constitutes a new unacceptable decision by the authorities of Damascus," a French Foreign Ministry statement said.

The Syrian uprising was inspired by the revolts and calls for reform sweeping the Arab world, and activists and rights groups say most of those killed have been unarmed civilians. An aggressive new military offensive that began with the Muslim holy month of Ramadan at the start of August has killed several hundred people in just one week.

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AP writer Zeina Karam contributed to this report. Bassem Mroue can be reached at http://twitter.com/bmroue and Zeina Karam can be reached at http://twitter.com/zkaram .

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