Security forces shot dead at least four civilians and wounded dozens more as thousands of Syrians gathered on Friday to call for regime change and to chide world powers for inaction in the face of a brutal crackdown.

Responding to a call under the slogan "Your silence is killing us," the protesters emerged from mosques in cities around the country, rights activists said.

At least 50,000 people turned out in the eastern city of Deir Ezzor to honour four people killed there on Thursday, and an estimated 300,000 attended the funerals of three of them, said Sami Abdel Rahman, of the Syrian Observatory for Human Rights.

Security forces shot dead one young man as protesters there were returning to their homes, Abdel Rahman said.

In Latakia, he said "a young man was killed and several others injured by gunfire from security forces who tried to disperse hundreds of demonstrators gathered at a square."

Another young man was killed by security forces in similar circumstances in Daraa, where security forces had also reportedly deployed in large numbers, said Abdel Karim Rihawi, who heads the Syrian League for the Defence of Human Rights.

And a fourth youth was killed at a checkpoint in the Damascus suburb of Kser, Abdel Rahman said.

Abdel Rahman said more that 500,000 people protested in Hama, but that there were no reports of violence there.

However, in the central city of Homs, security forces "fired on demonstrators in the Karam al-Shami district, wounding several people."

Rihawi also reported gunfire at Harasta near Damascus, with "more than 15 wounded."

Men under 50 were barred from entering mosques and all worshippers from taking in camera-equipped mobile phones in Daraa, one of the first places where Syrians rose up against President Bashar al-Assad's autocratic regime in March.

The protests, called for by Facebook group The Syrian Revolution 2011, are 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.

Only hours ahead of Friday's rallies, a dawn blast rocked an area near Homs, a flashpoint protest city, leaving a crater 15 metres (50 feet) across and oil gushing from a broken pipeline, SANA state news agency said.

"A subversive group targeted an oil pipeline near Homs at dawn with an improvised explosive device," SANA said.

Homs Governor Ghassan al-Adel said many residents reported hearing the explosion at around 4:00 am (0100 GMT).

"This terrorist operation, a subversive operation of the highest order, took place in a farming area, causing extensive damage," Adel said, quoted by SANA.

Deir Ezzor was the site of the only other pipeline attack in Syria since the unprecedented protests erupted in mid-March.

On July 13, an explosion hit a gas pipeline near the town of Mayadin in Deir Ezzor, the main oil- and gas-producing region in Syria, which produces 380,000 barrels of oil per day.

After the killings there on Thursday, about 3,000 people gathered outside the house of the new governor, Samir Othman al-Sheikh, to "demand an end to the killing," said Abdel Rahman.

Elsewhere, in Qatana, 25 kilometres (15 miles) south of Damascus, armed forces arrived in pickup trucks overnight and carried out arrests before searching for more protesters.

The sweep came as people took to the streets of Damascus and Qatana to protest after security forces killed 11 people on Wednesday in Kanaker, 50 kilometres southwest of the capital, activists said.

Among those said to have been detained were two prominent members of a national coordination committee for democratic change, Adnan Wehbe and Nizar al-Samadi.

Around another 100 people were arrested overnight in raids in Damascus, the Observatory told AFP.

The Avaaz non-governmental organisation says 1,634 people have died in the crackdown and 26,000 have been arrested, of whom 12,617 are still in detention.

This week it listed 2,918 people "who have been arrested by Syrian security forces and whose whereabouts are now unknown."

Syria blames the deaths on "armed terrorist groups" it accuses of wanting to spread chaos, and SANA said an armed band had fatally wounded a policeman in the easter town of Bukamal.

It added that there had been a pro-regime demonstration in the Damascus quarter of Bab Tuma to denounced the "terrorist act" committed near Homs.