By Renee Maltezou and Angeliki Koutantou ATHENS (Reuters) - Protesters threw petrol bombs at Greek police outside parliament in Athens on Wednesday as lawmakers prepared to vote on a painful austerity package that European creditors have demanded in return for a new bailout to keep the country in the euro. The clashes, the most serious Greek street violence in three years, erupted as Prime Minister Alexis Tsipras was struggling to contain a backlash in his own leftwing party against the deal that he was forced to accept after gruelling talks in Brussels earlier this week. More than half of the ruling Syriza party's 201-member central committee signed a statement rejecting the "humiliating" terms of the bailout, saying it was not compatible with the principles of the political left.