A dozen protesters have been arrested amid violent scenes at the Syrian embassy in London as the United Nations failed to agree a resolution despite a new wave of killing by the Damascus regime.

Scotland Yard said six people were arrested on suspicion of public order offences when a chanting crowd used missiles to smash several of the building's windows.

It came hours after five were detained for storming the embassy in protest against the brutal crackdown of the Syrian uprising and another on suspicion of assaulting a police officer.

Tensions were fuelled by an assault by Syrian security forces against the southern city of Homs which was reported to have killed upwards of 250 people.

Angry scenes in central London were mirrored in cities across Europe and the Middle East - including Libya and Tunisia where Arab Spring uprisings have already succeeded in removing regimes. Tunisia announced that it would withdraw its recognition of President Bashar Assad's leadership and expel the Syrian ambassador.

But the Homs bombardment, the bloodiest episode so far in the 11-month battle between Assad and pro-democracy protesters, did not unlock a diplomatic stand-off over UN action.

Britain accused China and Russia of encouraging Assad's "killing machine" after they vetoed a draft resolution aimed at ending the violence and securing a peaceful transfer of power.

Foreign Secretary William Hague said the twin veto was "an hour of shame" for the UN and that Russia and China had sided with the regime over the people of Syria and the Arab League.

"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," he said.

A crowd of around 150 tried to storm the London embassy in the early hours and a similar-sized crowd demanding the closure of the diplomatic post and throwing missiles returned later on.