US President Barack Obama has accused Syria of using "outrageous" force against anti-government protesters.
He condemned "in the strongest possible terms" Friday's violence, in which scores of protesters were killed, according to activists and witnesses.
Large crowds are expected to attend funerals for many of those killed, raising fears of further bloody confrontations.
Friday's death toll was the highest in a single day in five weeks of unrest.
Syria's state news agency said there had been a limited number of protests in some provinces and the violence was the work of armed criminal gangs.
It said security forces had used only tear gas and water cannon to prevent clashes.
Friday's bloodshed - which came a day after President Bashar al-Assad scrapped decades of emergency rule - brought strong international condemnation.
Human rights groups and activists gave death tolls ranging from just over 70 to more than 90, and one group said the death toll could reach 100.
Many of the deaths were reported to have occurred in the central city of Homs, the southern village of Izraa, and in a suburb of the capital, Damascus.
Video footage showed protesters scattering as they apparently came under fire.
Amnesty International said two boys aged seven and 10 were among those killed in Izraa.
"This outrageous use of violence to quell protests must come to an end now," Mr Obama said in a statement.
"Instead of listening to their own people, President Assad is blaming outsiders while seeking Iranian assistance in repressing Syria's citizens through the same brutal tactics that have been used by his Iranian allies."
Syria is widely believed to be a key part of an alliance that includes Iran, Hezbollah in Lebanon, Hamas in the Gaza Strip and other more radical Palestinian groups opposed to peace with Israel.
The US designates Syria a state sponsor of terrorism and already has sanctions in place.
UN Secretary-General Ban Ki-moon called for an independent investigation into the killings, while France said it was "extremely concerned" and condemned the violence.
"Light should be shed on these crimes and those responsible must be identified, arrested and brought to justice," foreign ministry deputy spokeswoman Christine Fages said.
Syrian authorities should "renounce the use of violence against their citizens", she added.
The BBC's Kim Ghattas reports from neighbouring Lebanon that the crowds across Syria on Friday are proof, if any was needed, that Mr Assad's concessions were belated and too symbolic.
The persistence of the demonstrations shows the growing strength and confidence of the protest movement, she says.
With foreign journalists unable to get into Syria, much of the reporting of the country has depended on footage distributed by opposition activists.
The official Syrian news agency said the army had found digital cameras containing short, fabricated videos depicting fake repression of protests on Friday.
It said armed gangs were carrying bottles of blood to be used in making fake films.
In their first joint statement since anti-government protests broke out, activists co-ordinating the mass demonstrations demanded the establishment of a democratic political system.