Activists called for a second day of mass rallies to protest alleged torture and death of a 13-year-old boy.
Opposition groups in Syria say the army has moved into towns and villages in the centre of the country, killing protesters.
Residents said on Sunday that troops have stormed the town of Talbiseh, where five people are said to have been killed.
Tanks have also pushed into Rastan, where it is claimed two people have been shot dead. Meanwhile, in Homs, Syria’s third largest city, there are reports that one person was killed when troops opened fire on two bus-loads of students.
Talbiseh is 10km north of Homs, where tanks shelled a main neighbourhood earlier this month.
The 10-week-old protests in Syria have evolved from a movement demanding reforms to a resilient uprising that is now seeking the overthrow of Bashar al-Assad, the country’s president who belongs to the minority Alawite community.
The Reuters news agency reported, quoting a prominent human-rights campaigner, that security forces killed at least 11 civilians and wounded scores on Sunday.
Razan Zaitouna, who is a rights lawyer, said by phone from Damascus, the largely peaceful capital, that the killings occurred in and around Talbiseh and Rastan.
The Syrian Observatory for Human Rights, which is based in Britain, said earlier on Sunday that it had the names of eight civilians killed.
“Soldiers are now all over Talbiseh. They are breaking into houses and arresting people,” one resident in the town of 60,000 said in a telephone interview.
The sound of bullets echoed in the background.
The official state news agency said four members of the security forces were killed in Talbiseh “while chasing armed terrorist groups … to detain them and present them to justice”.
The activists said Sunday’s attack on Rastan, Talbiseh and Teir Maaleh, in Homs, occurred after authorities cut all telecommunications in the area.
They said all roads leading to the two towns were closed off by security forces and soldiers.
The details could not be independently verified, as Al Jazeera is banned from entering Syria.
Earlier on Sunday, a mortar shell hit a school bus carrying children in Talbiseh, residents told Al Jazeera.
An ambulance trying to rescue them was also targeted, but the wounded students were eventually allowed to be transported to the nearest hospital in Deir Balba, they said.
Ages of the passengers ranged from six to 24 years, they said.
Separately, security forces fired on a bus carrying university students on their way from Hama to Homs, residents and opposition activists said.
The driver’s assistant was killed in fire, they said.
Separately, Mustafa Osso, a human-rights activist, said security forces opened fire in the early hours of Sunday at about 8,000 protesters in the northeastern town of Deir el-Zour, wounding several people.
He said there were protests overnight in different parts of the country, including the Damascus suburbs of Zabadani and Douma.
More than 1,000 people have been killed and 10,000 others arrested since the anti-government uprising began in Syria, according to rights groups.
Syrian authorities say 143 soldiers, security personnel and police have been killed.
Foreign journalists are barred from travelling inside Syria, making it difficult to report on the unrest and verify witness accounts.
The Assad government insists the unrest is the work of “armed terrorist gangs” backed by Islamists and foreign agitators.
It initially responded to the revolt by offering some concessions, including lifting the state of emergency in place for nearly five decades, but coupled this with a fierce crackdown.