You Say You Want a Revolution ... in Greece?

Rioting continues in Athens and other areas following the fatal shooting of a 15-year-old by police in Greece. According to the New York Times the Communist Party organized protests against the police, which later became violent.

Panagiotis Sotiris, 38, a spokesman for Uniting Anti-Capitalist Left, a coalition of leftist groups that helped take over the Athens Law School on Monday, told Reuters that the violence was not only connected to the killing, “but is a struggle to overthrow the government’s policy.”
“We are experiencing moments of a great social revolution,” he said.

Must all "great social revolutions" require violence?