Protests against President Bashar al-Assad's regime were held in most major towns, witnesses said, in pro-democracy demonstrations after the Muslim weekly prayers as on past Fridays.
At least 32 civilians were killed in the protest epicentre of Daraa, the London-based Syrian Observatory for Human Rights said, adding it had a list of names of those confirmed killed.
Military officials said four soldiers were also killed and two captured by "armed terrorists" in Daraa, though a rights activist in the southern town said the men had been killed defending protesters.
And in Homs, at least 15 civilians were killed in the industrial city, the Observatory said, updating an earlier toll from an activist.
The interior ministry said three members of the security forces were also killed in Homs.
"Three police officers... were shot dead today after being targeted by extremist terrorist groups when they performed their duties," the official SANA news agency quoted a ministry source as saying.
The Observatory said one person was also killed in the Mediterranean port city of Latakia.
The United States, meanwhile, blocked the assets of President Assad''s powerful brother, Maher, several other officials and of Syria''s intelligence services.
Ahead of Friday''s bloodshed, dissidents said security forces using live rounds and tear gas already killed more than 450 people since the pro-democracy protests erupted in mid-March.
Friday''s rallies come as the European Union met in Brussels to consider a wide range of sanctions against Damascus.
The call for mass demonstrations was issued on a Facebook page, The Syrian Revolution 2011, a motor of the protests in which demonstrators inspired by uprisings across the Arab world are seeking greater freedoms.
Assad''s embattled regime reiterated its running ban on demonstrations, despite having lifted a decades-old law barring them earlier this month, as the Muslim Brotherhood accused it of genocide.
Information Minister Adnan Mahmud told AFP the crackdown would continue, saying the "authorities are determined to restore security, stability and peace to the citizens.
The interior ministry appealed to Syrians not to join the protests and warned that unauthorised rallies would not be tolerated.
It called on "brother citizens to contribute in an effective way to stability and security... by not staging demonstrations or sit-ins for any reason without official permission."