Protest organisers have used the Facebook page, "Syrian Revolution 2011" to rally support against the regime of President Bashar al-Assad.
Activists said the move against communications coincided with a call for a Friday demonstration under the banner "Unity of the Opposition" to overthrow the regime, which they termed "a national duty."
They also voiced support for the opposition Syrian National Council SET up in August in Turkey at the initiative of Syrian Islamists. Last week the names of only half of its 140 members were revealed for security reasons.
Friday, the main Muslim day of prayer, is a rallying point for anti-regime protests, as thousands surge onto the streets to be met with a harsh security crackdown.
On Thursday, the official news agency SANA said: "Five members of the security forces have been killed and 17 wounded in an ambush mounted by some armed terrorist groups on the road from Tiba."
The area is in the southern Daraa province which was one of the main springboards of opposition to the authorities when protests began on March 15.
A customs official was also killed by "armed men" and two members of the security forces were wounded in Qseir south of Homs, SANA said.
Damascus does not accept that there is popular opposition to the authorities, saying instead that "armed gangs" and "terrorists" are trying to sow chaos.
Activists, meanwhile, reported more civilian bloodshed and arrests.
The Local Coordination Committees (LCC) reported three civilians killed in Homs, while in Daeel, in Daraa province, security forces shot at students, wounding some and arresting others.
It said the students were aged under 15 and that tanks had been stationed outside Daeel public high school.
The LCC, which has activists on the ground across Syria, is an active member of the Syrian National Council -- one of several opposition coalitions that has emerged since March to rally against the regime.
The Britain-based Syrian Observatory for Human Rights said that gunmen killed a civilian in Homs, but it was not clear whether this was a separate killing.
It also reported the arrest on Thursday of 57 people nationwide, including five pupils. According to the Observatory, 70,000 people have been detained since mid-March, with 15,000 still behind bars.
Prominent dissident Mohammed Saleh, who this week met members of a visiting Russian parliamentary delegation, was also arrested on Thursday by intelligence agents in Homs, the Observatory said.
Saleh reportedly told the Russian MPs about "the sufferings of the people of Homs," it added.
In Daraa province, security reinforcements arrived in the towns of Jezah and Tayebeh, the LCC said, and security forces deployed in the Nabu area in suburban Damascus.
According to another LCC report, security agents, troops and pro-regime militia in Homs have spread "a climate of terror" since Monday, "searching houses street by street, kidnapping many young people and taking them to a stadium that they had transformed into a prison."
Western governments have sought to increase pressure on Assad, and on Wednesday US President Barack Obama called on the UN Security Council to impose sanctions on Syria. Russia has resisted any such move.
Meanwhile the governor of Syria's central bank, Adib Mayaleh, said that the US embassy in Damascus had denied him a visa to attend a World Bank and International Monetary Fund meeting that begins on Friday in Washington.
"I find this decision mean-spirited," he told AFP.
In May, Washington began to punish Assad's regime and its supporters, freezing state assets and those of several officials because of the crackdown on dissent.
Syria's minister of economy and trade, Mohammed Nedal al-Sha'ar, announced that the government was suspending imports of cars and luxury goods in an effort to save foreign exchange, SANA reported.
He said that all goods subject to customs duties of five percent and more were included in the suspension.
The decision was made to "conserve the country's foreign reserves and to reallocate it to the lower income groups," he told reporters after the weekly cabinet meeting, SANA reported.
In Geneva, the Office of the UN High Commissioner for Human Rights said the death toll from the crackdown since March had risen to more than 2,700 people.
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