Tokyo (AFP) - Thousands of people rallied in Okinawa in southern Japan on Sunday in protest against a controversial US airbase on the island, as a two-decade-old bitter row over the relocation of the site drags on.

Okinawa is home to more than half of the 47,000 US service personnel stationed in Japan as part of a defence alliance, a proportion many islanders say is too high.

Futenma airbase has become emblematic of that ill-will since Washington announced plans to move it in 1996, hoping to ease tensions with the host community after the gang-rape of a schoolgirl by servicemen.

But locals have blocked the move to relocate the base, insisting the facility should go off-island instead, queering relations between Tokyo and Okinawa -- a once independent kingdom that was annexed by Japan in the 19th century.

"The government says we are to blame that the issue has stalled for 19 years and they tell us to find an alternative place (for the base relocation). That's outrageous," shouted the anti-US base mayor of Nago, Susumu Inamine.

"The government is thrusting their responsibility on us," Inamine told a packed 15,000-seat baseball stadium.

Organisers estimated that about 35,000 people also turned up for a rally in Naha, Okinawa's capital.

Deadlock has deepened recently after preparatory building work on the coast begun in the face of vehement opposition from the local government in Okinawa.

Prime Minister Shinzo Abe last month insisted the current re-location plan was "the only solution," while anti-base Okinawa governor Takeshi Onaga hit back saying that three recent popular votes in Okinawa all showed overwhelming opposition to the move.

"The current government is pushing the plan. Is it really a democratic country?" said protester Kiku Nakayama, 86, who as a teenager worked as a nurse for soldiers towards the end of World War II.

"We have to remove the risks of exposing Okinawa to war again," she said.

While most Japanese value the protection the US alliance gives them, especially in the context of Beijing's growing regional assertiveness, a sizable proportion of Okinawans want a dramatic reduction in their numbers.