Activists in Syria have vowed to stage a "day of defiance" after a crackdown by government troops.

"Today is going to be extremely important in Syria," Oxford University's Dr Alan George told John Humphrys as a "trial of strength" takes place between the regime and protesters.

"If there are significant demonstrations again today then the regime, in my opinion, has got real cause for worry about its ultimate survival," he said.

If the "bloody repression" conducted by the government is not "cowing" the population, he said, "I'm not sure what will".

"Thousands have been arrested and subjected to extraordinarily brutal torture... this is a regime that really is beyond the pale."

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