Syrian activists sent Al Jazeera a list naming 70 people from across the country who they said had been killed by security forces during the "Great Friday" protests.
Fifteen of the deaths took place in Izraa, near the flashpoint southern town of Daraa, according to the list.
Deaths were reported in Douma and Zamalka, near Damascus (see this video posted from an unknown source from Zamalka).
Other places where protesters were killed include Homs, Syria's third largest city, Moadamia and Daraa.
Demonstrators marching in peace were surprised by security forces' live ammunition, according to Hazem, a protester who spoke to Al Jazeera via phone from a Damascus suburb.
"Demonstrators were going with olive branches, it was peaceful" until they were "surprised by live ammunition from some security forces in one of the flats of the street", Hazem said.
The protesters took to the streets to mark what activists dubbed "Great Friday" - the biggest demonstrations against Bashar al-Assad's government to date.
Al Jazeera's Rula Amin reported from Damascus, which until now had been relatively calm, that the level of tension in the city on Friday marked a new point in the uprising.
"This day is turning into a very bloody day, probably the bloodiest since the protests started," she said.
A heavy security presence prevented protests from taking off in Damascus.
"Obviously the government want[s] to make a point, the capital is a redline and they will not allow the protests to reach the capital," she said.
Several witnesses, including medical professionals, told Al Jazeera that many of the injured were either being refused access to hospitals or were too scared to seek treatment.
A spokesperson for the ministry of information told Al Jazeera on Friday that security forces would fire on protesters only if they were fired upon first.
State television, meanwhile, aired a talk show where speakers blamed foreign media, including Al Jazeera, Al Arabiya and BBC Arabic, for inciting the protests.
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