Activists say Syrian security forces have killed at least 44 people, as large protests calling for a no-fly zone to protect civilians and soldiers deserting the army were held across the country.
The Syrian Revolution General Commission said in a statement that most of Friday's fatalities were in the protest hubs of Hama, in the north, and Homs in the centre.
Security forces forces encircled mosques before and after Friday prayers and made arrests, the UK-based Syrian Observatory for Human Rights (SOHR), said.
Hama and Homs have been at the front line of the pro-democracy protests against the government of Bashar al-Assad that have rocked Syria since mid-March.
The UN estimates more than 3,000 people, mostly civilians, have been killed in the violence.
"Despite the siege, the proliferation of checkpoints and the encirclement of mosques, people staged a mass demonstration in Kafr Nabl," a town in Idlib, near the Turkish border, demanding the "imposition of a no-fly zone," the SOHR said.
That call for the UN to impose a no-fly zone, blocking flights as they did in Libya, was echoed in Homs, the focus of military raids in recent weeks, where demonstrators came out in "most of the city's neighbourhoods," the observatory said.
Large protests were also reported in Deir Balaa and Hama, where clashes took place between suspected army defectors and members of the regular army and the security forces.
Troops also raided the northwestern town of Kafruma, arresting 13 people, including a woman and her 12-year-old son, the SOHR added.
In Maaret al-Numan, also in Idlib, the funeral of a soldier who defected and was shot dead on Thursday by security forces turned into a rally demanding the fall of Assad's regime, and demonstrators further east in Deir ez-Zor also came under fire as they streamed out of mosques, activists said.
Louay Safi, a member of the Syrian opposition, told Al Jazeera: "We hold President Bashar al-Assad responsible if things deteriorate further because, while he was talking about reform and making a lot of promises, nothing has been put in place.
He said the military and and special forces were keeping Assad in power.
"But the people have broken that wall, they are not fearful anymore and they are determined to gain control over their lives," he said.
The crackdown continues amid reports suggest neighbouring Turkey is harbouring Syrian anti-government fighters.
Turkey is said to be providing shelter to the Free Syrian Army, an armed opposition fighting against Assad.
The group is said to have claimed responsibility for killing nine Syrian soldiers