Syria said on Thursday that "terrorists" had killed five security force members and wounded 17 in the restive area of Daraa while pro-democracy activists said three civilians were killed in Homs, another hub of protest against President Bashar al-Assad.
Activists also said the authorities had blocked mobile phone signals and the internet in parts of Damascus province, at Saqba, Jisrin, Kafar Batna, Hamurieh and Ain Tarma.
Organisers of the protests against the Assad regime have used their Facebook page, "The Syrian revolution 2011," to rally support for their cause.
The activists said the move against communications coincided with a call by militants for a Friday demonstration under the banner "Unity of the Opposition", to overthrow the regime which they termed "a national duty."
The main Muslim day of prayer has seen weekly protests against the regime, with thousands of protesters surging into city streets to be met with a harsh crackdown by security forces.
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 this year.
Damascus does not accept any popular opposition to the authorities, saying instead that "armed gangs" and "terrorists" are trying to sow chaos in the country.
Local Coordination Committees (LCC) have joined a National Council of opposition which was formed at the end of August during an Islamist-inspired meeting in neighbouring Turkey.
On Thursday, the LCC reported that three civilians had been killed in Homs, while in Daeel, security forces opened fire on a group of students from different schools, with some being wounded and several arrested.
It said the students were aged under 15 and that tanks were now deployed in front of the Daeel public high school.
The Syrian Observatory for Human Rights also said that hidden gunmen had opened fire and killed a civilian in Homs. It was not clear whether this was a separate killing.
In Daraa province, security reinforcements had arrived in the towns of Jezah and Tayebeh, the LCC said, while security forces had deployed in Nabu area in suburban Damascus.
According to another LCC report, security agents, troops and pro-regime militia in Homs had spread "a climate of terror" since late last 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.
In Geneva, the Office of the United Nations High Commissioner for Human Rights said the death toll from the regime's bloody crackdown on protesters since mid-March rose to more than 2,700 people