Hundreds of riot police and protesters clashed Friday as construction of U.S. helipads resumed in Okinawa’s Northern Training Area, a key condition for the partial return to Japan of a large parcel of land being used by U.S. forces.

About 500 riot police faced off against 100 protesters while barricades were removed near the entrance to the construction site.

The move came on the same day that the central government filed a fresh lawsuit against Okinawa Gov. Takeshi Onaga over the long-delayed relocation plan for U.S. Marine Corps Air Station Futenma.

In addition to his opposition to the base plan, Onaga is also opposed to construction of the helipads because they can be used to support the Osprey, the controversial tilt-rotor aircraft nicknamed “the widow-maker” by critics familiar with its checkered safety history.

Nearby residents have also complained about the noise made by the Osprey and the environmental impact construction will have on the area.

The project is part of a 1996 pact between Tokyo and Washington to reduce the U.S. military’s footprint on the island, which many Okinawans have fiercely opposed for decades.

According to the deal, both the Japanese and the U.S. governments agreed that 4,000 hectares of the 7,800-hectare training area are to be returned on condition that six helipads are relocated to the remaining area. So far, only work on two helipads has been completed.

Vice Gov. Mitsuo Ageda summoned and lodged a protest with Koichiro Nakajima, director-general of the Defense Ministry’s Okinawa Defense Bureau, over the work resumption, saying the government should have met with residents first to explain the situation.

Meanwhile, the central government’s lawsuit against Gov. Onaga over the relocation of the Futenma base was filed at the Naha branch of the Fukuoka District Court. Oral arguments are scheduled to be heard on Aug. 5

Tokyo accuses Onaga of acting illegally by failing to follow its order to rescind his revocation of permission for landfill work to continue off Henoko in Nago, where the base is to be moved to.

“We had negotiated with Okinawa for years over the issue of land reclamation and received permission after complying with instructions given by the prefecture,” Defense Minister Gen Nakatani said on Friday. “We will continue to vigorously claim the illegality of Onaga’s revocation.”

Tokyo and Okinawa sued each over the relocation plan but withdrew the suits after agreeing to a court-mediated settlement by the Naha branch of the Fukuoka District Court in March that required Tokyo to halt construction and ordered Onaga to scrap his landfill revocation order.

Both parties also had to wait for the final ruling of an arbitration panel under the internal affairs ministry, which they could challenge in a court but whose verdict would be final, the agreement said.

Still, the panel’s decision in June was inconclusive and merely asked the two sides to keep talking. Okinawa decided not to take further legal action, hoping for continued dialogue with the central government.

But Suga met with Onaga on Thursday in Tokyo and told him Tokyo would sue Okinawa and the lawsuit would be based on that March settlement.

Information added by Kyodo