Britain accused Russia and China of encouraging the Syrian regime's "killing regime" after they vetoed a United Nations resolution seeking an end to the violent repression.
Foreign Secretary William Hague joined international condemnation of their stance amid protests at Syrian embassies around the world that led to six arrests in London.
Anger was fuelled by a renewed assault on the city of Homs by security forces overnight which is thought to have killed upwards of 240 people in the bloodiest day of the 11-month crackdown.
Mr Hague said: "Russia and China faced a simple choice today: would they support the people of Syria and the Arab League or not?
"They decided not to, and instead sided with the Syrian regime and its brutal suppression of the Syrian people in support of their own national interests.
"Their approach lets the Syrian people down, and will only encourage President Assad's brutal regime to increase the killing, as it has done in Homs over the past 24 hours."
The draft resolution, tabled by Morocco, did not impose sanctions or authorise military action and contained nothing that warranted opposition, he said.
Rocks and bottles were thrown at the London embassy, shattering windows, as police in riot gear attempted to quell heated scenes.
A Metropolitan Police spokesman said a number of people were arrested after gaining entry to the building while another individual was detained on suspicion of assaulting a police officer.