TOKYO, Aug 30 (Reuters) - Tens of thousands of protesters gathered near Japan's parliament building on Sunday to oppose legislation allowing the military to fight overseas, the latest sign of public mistrust in Prime Minister Shinzo Abe's security policy. In one of Japan's biggest protests in years - organizers put the crowd at 120,000 - people of all ages braved occasional rain to join the rally, chanting and holding up placards with slogans such as "No War" and "Abe, quit." Demonstrators swarmed into the street before parliament's main gate after the crowd size made it impossible for police, out in heavy numbers, to keep them to the sidewalks.