ATHENS (Reuters) - Greek police fired tear gas at protesters who hurled petrol bombs in Athens on Saturday during a rally marking four years since the killing of an anti-fascism rapper by an ultranationalist Golden Dawn supporter. The killing of Pavlos Fyssas, who performed under the stage name Killah P, had sparked protests across Greece and led to an investigation into Golden Dawn for evidence linking it to violent attacks. A trial of party members is continuing.   Clashes broke out on Saturday after dozens of hooded demonstrators broke off from a march of about 2,000 people, including activists and migrants, towards the Golden Dawn offices in central Athens.   Protesters shouted  Pavlos is alive! Crush the Nazis!  and held up banners demanding the party s offices are shut down and its members are jailed.  Police buses and officers in riot gear blocked the column from reaching the building.  A second rally to mark Fyssas s death anniversary on Sept. 18 has been called for Monday in the working-class neighborhood of Keratsini where he was stabbed to death. 