A heckler has thrown a Doc Marten boot at former PM John Howard during a speech at Cambridge University.Mr Howard was was preparing to speak to an audience of 400 at the Cambridge Union Society on the "leadership in the new century" when a student verbally abused him and accused him of being racist.A Cambridge student website, The Tab, said the man began his verbal attack when Mr Howard told the audience he had been the Australian prime minister for 12 years and listed his achievements while in office.The Tab said the man yelled: "I was ashamed to be an Australian for that entire time, racist."The website continued: "After this he was asked to the leave the union but he refused and took off his Doc Marten boot and attempted to throw it at the former head of government."Senior union members managed to intercept the boot and the man was escorted out."