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.