|Deir ez-Zor has seen heightened crackdowns on protesters since its governor was replaced last week [Reuters]
Security forces have fired at demonstrators in the southern city of Deraa as tens of thousands again took to the streets across Syria after Friday prayers to demand the downfall of President Bashar al-Assad, witnesses and activists said.
"They are firing at demonstrators in alleyways and people are running for cover. Protesters have been trying to avoid heavy security in main streets," a resident of Deraa told the Reuters news agency.
Security forces also used tear gas to try to disperse demonstrators in Deraa, witnesses said. There were no immediate reports of casualties.
Earlier on Friday, a bomb blast struck a major oil pipeline in western Syria, causing oil to spill into a nearby lake. State television said the explosion was a "terrorist'' attack by a group of "saboteurs".
It was the second incident involving an oil pipeline in a month, and the second time this week that authorities accused saboteurs of striking installations.
Oil sites targeted
Authorities said the pipeline carries crude from the oilfields in the oil-rich eastern Deir al-Zor to one of Syria's two oil refineries in the coastal town of Banias, the main point of export for Syrian oil. The second oil refinery is in the central city of Homs.
State TV said the blast hit near the western town of Talkalakh between Homs and Tartous, near the Tal Hosh dam, and left a 10-metre-deep crater. The TV said the "terrorist attack sought to cause oil to leak into
the dam's waters in order to damage agricultural crops in the area".
Sifian Allaw, the oil minister, said 1,500 barrels of crude oil leaked from the struck pipeline into the water behind the dam. The oil that gushed into the dam's waters caused a large spill, turning parts of the surface to black, he told The Associated Press.
Syria's oil exports are among the main earners of foreign currency for the government, especially now that the uprising has hit the tourism industry. The governor of Homs, Ghassan Abdel Aal, called the explosion a "first-class terrorist'' act, the AP reported.
The pipeline blast came as activists said security forces killed at least six people during overnight raids in Deir al-Zor province and the suburbs of Damascus, the AFP news agency reported.
Syrian authorities have unleashed a brutal crackdown in an effort to crush the revolt against Assad, and activists say more than 1,600 civilians have died since the protests erupted in mid-March. The government blames the unrest on terrorists and foreign extremists, not true reform-seekers.
The deaths in Deir al-Zor came amid reports of defected troops clashing with soldiers loyal to the ruling Baathist party.
"Tanks entered the city overnight, but there is talk of entire army units defecting. Electricity and communications have been cut," a resident, who declined to be named, told Reuters.
"Fighting is concentrating in the northwest of Deir al-Zor. It has been going on nonstop since 2am local time [2300 GMT]."
A significant number of people there have taken up arms to defend Assad's clampdown on anti-government protesters.
On Sunday, Assad replaced the civilian governor of Deir al-Zor province with Samir Othman al-Sheikh, a secret police officer, two days after roughly 500,000 people held demonstrations against the Assad government.
Last week the army surrounded the town of Albu Kamal on the easternmost edge of Deir al-Zor after 30 soldiers defected after the killing of protesters, residents said.
Syrian authorities have expelled most independent journalists since the uprising began, making it difficult to verify reports of clashes.
Syria readies for more
Meanwhile, Syrians attended mass protests after Friday prayers in what has become a weekly ritual of demonstrations and a brutal crackdown by security forces.
Opposition groups have dubbed Friday's protests "Your silence is killing us',' in an attempt to mobilise large sections of the population that have not yet joined the protests, as well as Arab leaders who have remained silent on the crackdown in Syria.
In the Damascus suburb of Qatana, security forces armed with machine guns and other weapons arrived in pickup trucks overnight and carried out the arrests before searching for more protesters.
The sweep came as people took to the streets to protest after security forces killed 11 people on Wednesday in Kanaker, 50km southwest of the capital, said human rights activists.
Among those said to have been detained were two prominent members of a national co-ordination committee for democratic change, London-based Syria Observatory for Human Rights said.
"Security forces ... arrested two known Syrian opposition figures Adnan Wehbe and Nizar al-Samadi," it said, adding that their fate "remains unknown".
Wehbe is a leader of the Democratic Socialist Arab Union Party and Samadi is a well-known Islamic personality from Douma, a protest hub outside Damascus.