Activists say Syrian troops have shelled two rebel-held neighborhoods in the central city of Homs.
The Britain-based Syrian Observatory for Human Rights says today's bombardment lasted for about an hour. It says there were no reports of casualties.
The shelling comes two days after a ceasefire brokered by international envoy Kofi Annan went into effect.
Violence reported over the last two days is much less than the daily norm of clashes and shellings before the truce.
Activist Tarek Badrakhan, who is based in Homs, and the Observatory said the shelling targeted the neighborhoods of Jouret el-Shayah and Qarabees.
The truce is at the center of Annan's six-point plan to stop the bloodshed and launch talks between the regime and the opposition.
Activists said security forces killed at least six people Friday, a lower-than-usual toll. The rallies, described as some of the largest in months, stretched from the suburbs of Damascus to the central province of Hama, Idlib in the north and the southern province of Daraa, where the uprising began in March 2011.
"Come on, Bashar, leave!" the crowd shouted in Daraa, linking arms and stomping their feet to the beat of a drum in a traditional Arab folk dance, according to a video posted online by activists.
The protests might have been far larger had President Bashar Assad's regime not violated a key aspect of the truce by keeping troops, tanks and snipers in population centers instead of pulling them back to barracks. The presence of plainclothes agents of the feared Mukhabarat security service also had a chilling effect on some of the gatherings in Damascus, the capital, and elsewhere.
The demonstrations were a critical test of the cease-fire, which went into effect at dawn Thursday, because they challenged the government's commitment to avoid the kind of attacks that have made Syria one of the bloodiest conflicts of the Arab Spring revolts.
Regime forces tried to block protesters from occupying main squares out of fear they will form a sit-in akin to Cairo's Tahrir Square, where hundreds of thousands of people camped out for days in an extraordinary scene that drove longtime Egyptian leader Hosni Mubarak from power.
Many world leaders expressed doubt that the truce would endure in a country where 9,000 people have been killed during the 13-month uprising, according to U.N. figures.
"I don't believe Bashar Assad is sincere," French President Nicholas Sarkozy told French television station i-Tele on Friday. Observers must be sent to find out what's happening."
A team of UN observers was on standby to fly into Syria and monitor the truce, but the mission still needed approval from the Security Council. Russia's UN Ambassador Vitaly Churkin told reporters the text was more complicated than he expected and that more negotiations would be needed, but he said his government also wanted to act quickly to get observers on the ground.
Russia has been one of Syria's strongest allies, shielding Assad from international condemnation at the UN out of fear that it would open the door to possible NATO airstrikes like those which helped topple Libya's Moammar Gadhafi.
President Barack Obama has ramped up US aid, including communications equipment and medical supplies, to Syria's opposition in hopes of accelerating Assad's downfall of Assad, officials said Friday.
The president signed off on the package last week, US officials said on condition of anonymity because of the sensitivity of the situation. They declined to outline all forms of American assistance because of the danger anti-Assad protesters have faced over the last year.
Despite the hitches in the cease-fire plan, Syrians poured into the streets yesterday. A particularly large protest of many thousands was reported in the sprawling Damascus suburb of Douma, where the regime conducted sweeping arrest raids in the days before the truce.
"It was an example of what a large peaceful protest can be like when the government does not intervene and fire on people," said local activist Mohammed Saeed.
But there were violent eruptions, as well, as security forces fired live rounds, tear gas and beat protesters with clubs in some areas.
Activist Adel al-Omari said security forces opened fire at protesters in the southern village of Nawa as they gathered in a central square, killing at least two.
"Once they gathered in the village's main square they came under fire," al-Omari said.
The Britain-based Syrian Observatory for Human Rights, which has a network of sources on the ground, said the dead also included two marchers who were in a crowd trying to reach the main Assi Square in Hama, an opposition stronghold.
Troops and pro-government militiamen known as shabiha beat protesters chanting anti-government slogans as they tried to leave a mosque in the Damascus neighborhood of Qadam, said the Local Coordination Committees, an activist network. In Syria's largest city, Aleppo, troops fired tear gas at marchers gathering outside the Grand Mosque, the group said.
The LCC put the nationwide death toll at 13 protesters, while the Syrian Observatory for Human Rights said at least six were killed. Late Friday, both groups said explosions and gunfire were heard in districts of the central city of Homs but that it was not immediately clear what was happening.
The regime restricts access of foreign observers, including journalists, making it difficult to verify death tolls and other claims independently.
The uprising began last year with mostly peaceful protests against the Assad family dynasty, which has ruled Syria for more than four decades. But the government's violent crackdown fueled an armed insurgency as army defectors and protesters began fighting back.
The rebel Free Syrian Army, which includes army defectors, has said it will observe the cease-fire. But the opposition is not well organized, and there are growing fears of groups looking to exploit the chaos.
Syria's state-run television said gunmen shot and killed Army Maj. Moussa Tamer al-Youssef while on his way Friday to his unit in Hama, saying the assassination was proof the opposition was not interested in a political solution to the crisis.
The truce is at the center of international envoy Kofi Annan's six-point plan to stop the bloodshed and launch talks between the regime and the opposition.
Western powers have condemned the violence, but they have few options to help stop it. They have all but ruled out NATO-style military intervention, in part because the conflict is so explosive and could spark a regional war.
Earlier Friday, Syrian troops clashed with rebels near Turkey, raising fears that the conflict could spill across the border. Syria's state-run news agency said authorities foiled an infiltration attempt by "armed terrorist groups" from Turkey and that the group fled back to Turkey.
Annan's spokesman played down the incident.
Clashes between Syrian troops and opponents are "not unusual," Ahmad Fawzi said. "Sometimes, in situations like this, the parties test each other."
"We hope both sides will sustain this calm, this relative calm," he added. "We are thankful that there's no heavy shelling, that the number of casualties are dropping, that the number of refugees who are crossing the borders are also dropping."