Foreign Secretary William Hague has condemned the "shocking" Syrian security force raids that have killed between 70 and 120 people.
Tanks stormed the city of Hama before dawn in a brutal crackdown on protests over President Bashar Assad's rule.
Bodies lay scattered on the streets following rounds of shelling and gunfire, according to residents.
Raids were also reported in southern Syria and in the suburbs of the capital Damascus.
It comes ahead of the start of the Muslim holy month of Ramadan on Monday.
Mr Hague said there was "no justification" for the attacks and called on the President to "stop this assault on his own people".
He said: "I am appalled by the reports that the Syrian security forces have stormed Hama with tanks and other heavy weapons this morning killing dozens of people. Such action against civilians who have been protesting peacefully in large numbers in the city for a number of weeks has no justification.
"The attack appears to be part of a co-ordinated effort across a number of towns in Syria to deter the Syrian people from protesting in advance of Ramadan.
"The attacks are all the more shocking on the eve of the Muslim holy month. President Bashar is mistaken if he believes that oppression and military force will end the crisis in his country.
"He should stop this assault on his own people now."
