Syrian troops are reported to have deployed in the suburbs of Damascus and in towns to the north of the capital.Protesters have said they will go ahead with a nationwide "day of defiance".
A reliable source told the BBC that protesters and security forces had clashed near a Damascus mosque, with stones thrown and tear gas fired.
More than 500 Syrians are thought to have been killed during attempts to quell seven weeks of protests against President Bashar al-Assad's government.
At least 2,500 people have been detained as part of a violent crackdown, which the US has described as "barbaric".
Troops and tanks have withdrawn from the southern city of Deraa, where a human rights group says security forces carried out a 10-day "massacre".
The government has said the military has been acting, especially in Deraa, to quell armed gangs that have been terrorising residents.
Troops have gathered in the Damascus suburbs of Irbin, Saqba, and Douma.
One reliable source in the city said there was a great deal of security outside one mosque. Another source told the BBC that security forces near a mosque had fired tear gas at protesters who threw stones.
Witnesses said there was a protest by dozens of people in the Damascus district of Midan. One told the Reuters news agency it been broken up plainclothes security personnel.
In the town of Tal, north of Damascus, witnesses told Reuters news agency that security forces had fired at protesters.
Earlier, extra troops were deployed in Homs, Syria's third city and scene of a bloody crackdown last month, and the central town of Rastan.
In Baniyas, a town on the coast that has seen unrest since anti-government protests began in mid-March, four armoured personnel carriers, several tanks and a bus carrying soldiers were seen by one eyewitness quoted by the Associated Press.
Hundreds of families were said to be fleeing the area, fearing that Baniyas - like Deraa - could come under siege.
"It looks like they are preparing to attack the town, like they did in Deraa," one activist told the AFP news agency by telephone from the town.
Across Syria, protesters are calling for greater political rights and personal freedoms. Some are calling for the downfall of the regime of President Assad.
People are expected to gather again after Friday prayers, which have become a regular focal point for protests in the Arab world in 2011.
The unrest in Syria poses the most serious challenge to Mr Assad since he succeeded his father, Hafez, in 2000.
Foreign journalists are not allowed to enter the country, so it is difficult to verify the reports of deaths.
But one doctor, who planned to join those demonstrating, said the "indiscriminate killings and inhumane arrests have generated total disgust among the average Syrian".
"Soldiers with rifles no longer deter people. The propaganda that this regime is the only guarantor of stability no longer washes," he was quoted as telling the Reuters news agency.
Appeals A Red Cross team has arrived in Deraa carrying medical supplies and humanitarian aid.
The head of the Damascus office of the International Committee of the Red Cross (ICRC) said Deraa "is a priority for us, because it is the city that has been hardest hit by the ongoing violence".
A UN humanitarian team is also expected to visit the city, following an appeal to President Assad by UN Secretary General Ban Ki-moon.
In Washington, state department spokesman Mark Toner said the US continued to "press Assad's regime to desist in its violent behaviour".
"We abhor the violence there," he told reporters.
"I think I called it barbaric, the measures that were taken the other day against the citizens of Deraa, and we urge Syria to end these kinds of actions against innocent civilians who are simply expressing their aspirations for a democratic future."
The Damascus Centre for Human Rights Studies (DCHRS) labelled the killings in Deraa a "massacre", saying snipers and anti-aircraft machine guns had been used to fire on unarmed civilians. Recent amateur video appeared to show dozens of unarmed protesters being shot and bleeding to death on the streets.
The Syrian government says it is taking action against "elements of terrorist groups... to restore security, peace and stability".
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