The vote is seen as highly symbolic but is also non-binding, raising questions about what effect it will have, even if opponents of the move, including Okinawa Governor Denny Tamaki, prevail.

The ballot will ask residents whether they support a plan to reclaim land at a remote coastal site for the relocation of the Futenma base from its current location in a heavily-populated part of Okinawa.

The referendum was initially planned as a yes-no vote on the move, but a "neither" option was added after several cities with close ties to the central government threatened to boycott the vote.

Although the referendum is not legally binding, "it is significant that people in Okinawa can express their will through the vote," said Jun Shimabukuro, a professor at Ryukyu University in Okinawa.

"It can be a test to gauge if democracy is working in Japan," Shimabukuro told AFP.

The relocation of Futenma to Nago, 50 kilometres (30 miles) away, was first agreed in 1996 as the US sought to calm local anger after US servicemen gang raped a local schoolgirl.

But the plan has long been stalled in part over local opposition.

- 'Making a fool of Okinawa' -

The Futenma base has stoked tension with local residents over problems ranging from noise and military accidents to crime involving base residents.

Prime Minister Shinzo Abe's government says the relocation will address those concerns, but many in Okinawa want the base relocated elsewhere in Japan.

They argue that the region bears a disproportionate burden when it comes to hosting US military troops in the country.

Okinawa accounts for less than one percent of Japan's total land area, but hosts more than half of the approximately 47,000 American military personnel stationed in Japan.

Anti-base rallies have been staged daily in Okinawa since campaigning for the referendum began in mid-February.

But the vote has not stopped reclamation work at Nago, with construction workers continuing to shovel dirt into the ocean offshore with bulldozers.

"We hope the referendum will boost the momentum of our fight," demonstrator Masaru Shiroma told AFP, as more than 100 fellow activists tried to block trucks entering the construction site on Friday.

"The government is making a fool out of Okinawa," he added.

"Our fight will continue even after the referendum, and will go on until the government gives up the plan."

- 'Precious votes' -

Polling stations open in most parts of Okinawa at 7:00 am (2200 GMT on Saturday), though some more remote parts of the chain of subtropical islands started voting earlier in the week.

The ballot closes at 8:00 pm with exit polls expected soon after and official results from as early as midnight.

Okinawa's governor is required to "respect" the vote's outcome if it is approved by at least a quarter of eligible voters -- around 290,000 votes.

Tamaki has urged residents to turn out and cast their "precious votes" in the poll.

A opinion poll by the Asahi Shimbun newspaper this week found 59 percent of people in Okinawa oppose the reclamation while 16 percent support it.

The survey also found 80 percent of respondents want Abe's government to respect the results.

But there has been little sign Abe's government will shift course if the vote goes against the move, with Chief Cabinet Secretary Yoshihide Suga saying there were no plans to halt the relocation regardless of the outcome.

Japan's military alliance with the United States is seen as a key partnership, and Okinawa's location near Taiwan has long been viewed as having huge strategic importance for US forward positioning in Asia.