Syrian troops shot dead three people who stoned a massive military convoy on Saturday as it headed to quash growing anti-regime dissent in the eastern oil hub of Deir Ezzor, an activist said.

But two human rights organisations from Syria said security forces killed at least 20 people and wounded 35 others on Friday as hundreds of thousands of demonstrators turned out for anti-regime protests.

Meanwhile a man identifying himself as a Syrian army colonel said in Nicosia that he had defected and has ‘hundreds’ of troops under his command ready to confront the regular army in Deir Ezzor.

Rami Abdel Rahman of the Syrian Observatory for Human Rights said a convoy of about 60 military vehicles including tanks, personnel carriers and trucks crammed with soldiers deployed in Deir Ezzor, mainly at the governor's office.

"The troops opened fire to frighten residents after reaching the governor's office," Abdel Rahman said, quoting witnesses in the eastern city.

There were mounting fears the army was preparing to crack down on Deir Ezzor, increasingly at the forefront of more than four months of protests against the autocratic regime of President Bashar al-Assad.

"People chanted 'Allahu akbar' (God is greater) in the city as a warning against a huge military operation and residents began setting up sand barricades to prevent the army from spreading into the city," Abdel Rahman said.

As the tanks and military vehicles rumbled towards Deir Ezzor they were confronted by an angry mob from the nearby village of Tibneh who hurled stones at the troops to stop their advance.

"The soldiers opened fire on them to disperse them, killing three people," the activist said by phone from Britain.

Tibneh is about 40km (32 miles) west of Deir Ezzor, where hundreds of thousands gathered Friday for the funeral of three people killed by security forces the previous day, according to activists.

Deir Ezzor, the main oil- and gas-producing region in Syria, which produces 380,000 barrels of oil per day, has seen almost daily demonstrations against the regime.

Protests have swelled on Fridays -- the day of rest when devout Muslims gather in mosques for key noon prayers -- across the city and the province also called Deir Ezzor.

On July 13 a rare explosion hit a gas pipeline near the provincial town of Mayadin -- in the first such act of defiance against the regime since pro-democracy protests broke out in mid-March.

Another blast on Friday rocked an area near Homs, a flashpoint protest city in central Syria, only hours before nationwide anti-regime protests.

That blast left a crater 15 metres (50 feet) in diameter and oil gushing from a broken pipeline, according to the state news agency SANA which blamed the attack on ‘a subversive group.’

Riad al-Asaad, who identified himself as a colonel who defected from the Syrian army, warned authorities against carrying out any operation in Deir Ezzor.

"I warn the Syrian authorities that I will send my troops to fight with the (regular) army if they do not stop the operations in Deir Ezzor," Al-Asaad said.

"I am the commander of the Syrian Free Army," he said, adding that he commanded "hundreds" of troops. The claim could not be independently verified.

The caller said he was speaking from inside Syria ‘near the Turkish border’, but declined to elaborate.

Syrian human rights organisations version

The deaths were reported on Saturday by two Syrian human rights organisations, one of them also saying that hundreds of people were arrested by security forces in Damascus.

"Nineteen martyrs fell on Friday," the National Organisation for Human Rights said in a statement in Nicosia.

"The Syrian authorities had decided to go ahead and kill protesters during the day marked by demonstrations dubbed 'Your silence is killing us'," it said.

The toll included one person killed in Damascus and seven in the region around the capital, including five in Kiswah and two in Douma, said Ammar Qorabi, who heads the human rights group.

Another three were killed in the flashpoint southern town of Daraa, three more died in the eastern city Deir Ezzor, two others in the nearby town of Bukamal, and one in the western coastal city of Latakia.

One person was killed in a village in the central governorate of Homs and another in the flashpoint city of Hama, also in central Syria, said Qorabi, adding that he had a list of names for those killed on Friday.

For its part, the Syrian Observatory for Human Rights reported 12 civilians were killed and 35 were wounded on Friday -- including 11 whose names figured on the list provided by Qorabi's group.

The 12th victim, a young man, was shot dead in Qadam, a Damascus neighbourhood, the Britain-based Observatory said.

"Soldiers and a large number of security agents entered Qadam at 3:00am on Friday (0000 GMT) and cordoned off the area," said the head of the group, Rami Abdel Rahman.

"More than 500 people were arrested during the operation, and one young man was killed at a security roadblock," he said.

Abdel Rahman said the authorities also banned public funerals in the district, and said the young man was quickly buried in the presence of security officials to forestall any demonstrations.

"The army put up barricades at all entrances to the area, and heavily armed members of the security forces carried out searches and made arrests," he said, adding they had lists of names of people hostile to the regime.

"Security forces broke down doors when they weren't opened quickly enough, and then they posted people on the roofs of houses during the four-hour operation," he said.

The security forces were deployed en masse across the country for another day of protest after the Muslim weekly Friday prayers against the regime of President Bashar al-Assad.

Friday's protests, called for by Facebook group The Syrian Revolution 2011, were aimed at putting pressure on the rest of the world to act in the face of the deadly crackdown on dissent by Assad's government.

"Where are you, defenders of freedom?" and "Enough of your silence ... silence is a shot in our chests," activists said on the website, a driving force behind the protest movement.

Activists said that hundreds of thousands swarmed the streets across Syrian on Friday, with half a million marching in the flashpoint city of Hama.

Since the protest movement emerged in March, 1,888 people have been killed, 1,519 of them civilians and 369 members of the security forces, according to the Observatory.

In addition, more than 12,000 people have been arrested and thousands more have fled, according to non-governmental groups.