Update at 3:16 p.m. ET: ABC News reports that President Obama issued an execution order today with new sanctions against Syria. The actions freeze the assets and ban Americans from doing business with three individuals -- Mahir Al-Asad, the Brigade Commander in the Syrian Army's Fourth Armored Division; Ali Mamluk, director of the General Intelligence Directorate; and Atif Najob, the former head of the Syrian Political Security Directorate for Daraa Province, site of the bloodiest clashes of the six-week-old popular uprising, ABC's Jake Tapper reports.

The same sanctions are also imposed against two groups: the Syrian General Intelligence Directorate and the Islamic Revolutionary Guard Corps – Qods Force.

Update at 3:09 p.m. ET: Security forces opened fire Friday on demonstrators trying to break an army blockade on the southern city of Daraa, while thousands of others across Syria defied a protest ban and denounced President Bashar Assad's harsh crackdown on a six-week uprising.

At least 42 people were killed, including 15 in the march on Daraa, according to witnesses and a human rights group, The Associated Press reports.

The protesters in cities across Syria - including the capital of Damascus - called for Assad's ouster, with some chanting "We are not afraid!"

Human rights activist Mustafa Osso said 42 people were killed, but the death toll could rise. His human rights group, based in Syria, compiles casualty tolls from the crackdown

Update at 2:22 p.m. ET: The French news agency, AFP, reports that the U.S. is imposing sanctions against teh brother of Syrian President Bashar al-Assad. The BBC says President Obama will impose sanctions on three top Syrian officials, Syria's intelligence agency and Iran's Revolutionary Guard.

Update at 1:02 p.m. ET: A Syrian human rights group reports that at least 24 people were killed today in clashes between security forces and protesters in several Syrian cities,The Associated Press reports.

Update at 1 p.m. ET: Reuters, quoting a medical source, reports that a hospital near the southern city of Deraa received fifteen bullet-riddled bodies of villagers killed when security forces opened fire on thousands of protesters, according to Al-Jazeera.

Update at 11:15 a.m. ET: Thousands chanting "We are not afraid!" demonstrated across Syria today, The Associated Press reports..

Al-Jazeera reports as many as 15 people killed in clashes around the country.

The AP quotes human rights activist Mustafa Osso, who compiles reports from sources around the country, as saying at least two people were killed and many others were wounded in a day of protests.

The deaths were near the northern city of Idlib and in the coastal city of Latakia — the heartland of the ruling elite, the activist reports.

Security forces also opened fire on demonstrations in the capital of Damascus and in Latakia, wounding at least five people.

State-run television says a military post in Daraa, the center of the six-week-old popular uprising, was stormed by armed men who killed four soldiers and captured two.

Troops also were reported to have fired on villagers trying to break through a blockade of the city near the Jordanian border.

Update at 10;47 a.m. ET: An estimated 15,000 protesters, many calling for the toppling of the regime of Syrian President Bashar al-Assad, marched in Damascus through the conservative Sunni neighbourhood of Midan, Al-Jazeera reports, quoting several witnesses.

Update at 10:04 a.m. ET: Al-Jazeera reports that at least four people have been killed after Syrian security forces fired on demonstrators in the coastal city of Latakia. In addition, as many as 11 people have died in clashes in the southern city of Daraa, including four members of the military, Al-Jazeera reports.

Al-Jazeera posts video of purported demonstration today in the town of Deir al-Zour.

Original post: Syrian security forces opened fired today on demonstrators in the coastal city of Latakia as thousands of protesters turn out in several town following Friday prayers, The Associated Press reports.

The AP quotes witnesses as saying five people were wounded.

Al-Jazeera reports that troops have cut off and surrounded the southern city of Daraa, which has been the center of a six-week-old popular uprising against the regime of President Bashar al-Assad.

Al-Jazeera quotes on witness in the city of 300,000 as saying it is "completely surrounded by tanks and armed troops."

"There are snipers on the roofs of government buildings and tall buildings," the witness tells Al-Jazeera. "They are hiding behind water tanks and some are even hiding in the minarets of mosques."

Syria's state-run TV says "armed terrorists" have attacked a military post in Daraa, killing four soldiers and capturing two.