The US said Syria was trying to "repress and intimidate demonstrators", while UN chief Ban Ki-moon urged Damascus to exercise restraint.
Witnesses said dozens of people had been killed during the protests - this cannot be independently confirmed.
The rallies came a day after Syria said it would consider political changes.
Protests were staged in towns and cities across Syria, including the capital Damascus.'Brutal response'
In the US, White House spokesman Jay Carney said Washington was deeply concerned at what was happening in Syria.
"We strongly condemn the Syrian government's attempts to repress and intimidate demonstrators," Mr Carney said late on Friday.
Meanwhile, UN Secretary General Ban Ki-moon telephoned Syrian President Bashar al-Assad to urge "maximum restraint".
Mr Ban also stressed that Damascus had an obligation to respect the fundamental rights of its citizens.More