CAMP FOSTER, Okinawa -- Tens of thousands of Japanese protesters gathered Sunday near a U.S. Marine Corps base on Okinawa to oppose next month's planned deployment of MV-22 Osprey aircraft to the island. Local government officials condemned the airplane-helicopter hybrid as defective, while red-clad opponents waved signs calling for the U.S. to halt plans to replace aging Marine Corps helicopters at the Futenma air station with Ospreys. The rally in Ginowan City was one of the largest anti-U.S. military demonstrations on the island in recent years and underscored the persistent skepticism here over the Osprey's safety, despite U.S. and Japan probes that have so far ruled out mechanical failures in two recent accidents that killed or injured crew members. An emergency landing Friday of an Osprey near Marine Corps Air Station New River in North Carolina due to an oil leak added fresh fuel to the protest. Masaharu Kina, speaker of the Okinawa prefectural assembly, claimed the incident is further evidence the aircraft is structurally flawed and should not be allowed to fly in Japan. "Here on Okinawa, the military is about to carry out the deployment plan, ignoring our overwhelming voices against it," Kina said. The deployment is part of the Marine Corps' global effort to upgrade its helicopter fleet , which has relied on dual-rotor CH-46 Sea Knights for transport since the Vietnam War. The service has repeatedly said the Osprey has proven to be one of its safest aircraft since it was first put into service in