Syrian troops fired live ammunition and tear gas at protesters in two cities and killed at least two people on Friday, activists said, as tens of thousands filled the streets to urge others who have remained on the sidelines to join them in pushing for the regime's ouster.

The protests, which have become a weekly ritual after Friday's Muslim prayer services, came hours after a bomb blast struck a major oil pipeline in western Syria, causing oil to spill into a nearby lake. State television called the explosion a terrorist attack by a group of "saboteurs."

The government of President Bashar Assad has sought to discredit those behind more than four months of protests against his rule by saying they are terrorists and foreign extremists, rather than true reform-seekers. At the same time, authorities have waged a brutal crackdown aimed at crushing the revolt, killing more than 1,600 civilians since the protests began in mid-March, activists say.

Opposition groups dubbed Friday's protests "Your silence is killing us" in an attempt to mobilize sections of the population that have not yet joined the protests and Arab leaders who remain silent about the crackdown in Syria.

The London-based Syrian Observatory for Human Rights said troops on Friday opened fire on protesters in the Mediterranean city of Latakia, killing at least one demonstrator.

The Local Coordination Committees, which also tracks anti-government protests, confirmed the death, and said another person was killed in a shooting by security forces during a protest in the southern city of Daraa.

Troops beat up protesters in the northwestern coastal town of Banias and fired tear gas in several other locations. There were also marches in the countryside around Damascus despite an intense crackdown there in the past few days.

The scale of the bloodshed has prompted some soldiers to defect to the anti-government side and even turn on their fellow soldiers, according to witness accounts.

One such fight took place Thursday and Friday in the oil-rich eastern city of Deir el-Zour, an activist living there said. Tanks and troops moved into neighborhoods where anti-regime protests were common, and some soldiers among the 7th Battalion of the 137th Brigade defected and clashes with loyal soldiers, the activist said, speaking on condition of anonymity for fear of reprisals.

Security forces there shot dead at least three civilians after residents tried to stop the troops' advance by placing roadblocks and stones in their path, according to several rights groups. The Deir el-Zour activist put the number of dead as high as seven.

Last week, Assad had fired and replaced the governor of Deir el-Zour after massive anti-government demonstrations in the area.

The regime has banned nearly all foreign media and restricted coverage of the uprising, making it nearly impossible to independently verify events on the ground.

The pipeline explosion was the second incident to cause damage to an oil line in a month and the second time this week that authorities accused saboteurs of striking key infrastructure.

The pipeline carries crude from the oil fields in Deir el-Zour to a refinery in Banias, the main point of export for Syrian oil.

State TV said the blast hit near the western town of Talkalakh near the Tal Hosh dam, and left a 33-foot (10-meter) 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."

Oil Minister Sufian Allaw said 1,500 barrels of crude oil leaked from the pipeline into the water. He told The Associated Press that the pumping of oil was transferred to another pipeline without interruption in the flow.

The area of Friday's blast, Talkalakh, is an opposition stronghold near the border with Lebanon that was overrun by army tank units, security forces and pro-regime gunmen in May after weeks of protests calling for the president's ouster. Rights activists say around 35 people died in the crackdown in Talkalakh.

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. Last year, tourism accounted for roughly 12 percent of GDP and brought in $8 billion in hard currency.

Syria produces about 350,000 barrels of oil per day as well as natural gas.

On July 13, a blast and a fire struck a natural gas pipeline in eastern Syria. Some rights groups said it was an attack but the Oil Ministry denied any explosion and said a fire erupted on a pipeline that was under maintenance.

And last Saturday, authorities said saboteurs tied to the country's uprising caused a passenger train to derail in central Syria, but opposition figures dismissed the accusation.