GINOWAN, Japan (Reuters) - Thousands of Japanese gathered in sweltering heat on the southern island of Okinawa on Sunday to demand that a U.S. Marine base be moved out of the region, days ahead of a visit by President Barack Obama.

The row over the re-siting of the Futenma air base threatens to stall a realignment of the 47,000 U.S. military personnel in Japan and sour defense ties between the two countries, seen as key in a region home to a rising China and an unpredictable North Korea.

It could also prove a domestic headache for Prime Minister Yukio Hatoyama, whose support ratings have slipped since his landslide election victory in August.

“Okinawa’s future is for us, the Okinawan people to decide,” Ginowan mayor Yoichi Iha told a supportive crowd which spilled out of an open-air theater by the beach. “We cannot let America decide for us.”

Organizers put the number of protesters at 21,000.

Under a 2006 U.S.-Japan agreement, the Futenma Marine base in the center of the city of Ginowan is set to be closed and replaced with a facility built partly on reclaimed land at Henoko, a remoter part of the island, by 2014.

The deal, which Washington wants to push through after years of what a military official called “painful” negotiations, is part of a wider plan to re-organize U.S. troops and reduce the burden on Okinawa by moving up to 8,000 Marines to Guam.

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U.S. Defense Secretary Robert Gates has urged Japan to approve the plan ahead of Obama’s visit, which is scheduled to start on November 12.

EMOTIVE ISSUE

Hatoyama, who has vowed to build a more equal relationship with the United States, said in the run-up to his August election victory the base should be moved off the island.

That view was supported by 70 percent of Okinawa residents in a poll published this month by the Mainichi newspaper.

“I think getting rid of Futenma would be a good starting point for the removal of all the U.S. bases from Okinawa,” said a 60-year-old woman at Sunday’s protest, who gave her name only as Shinzato.

Okinawa, controlled by the United States until 1972, makes up only 0.6 percent of Japan’s land mass, but hosts about half the U.S. troops in Japan. Those who live near the bases complain of noise, crime, pollution and accidents.

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“It’s such a wonderful place. It makes no sense to build it here,” said Hiroshi Ashitomi, a long-time anti-base campaigner.

Environmentalists are anxious to protect marine life including coral and rare dugongs in nearby waters.

Others have different priorities.

“Nature is important, but the primary responsibility of a politician is to protect people’s lives and property,” said Kosuke Gushi, a regional assemblyman with the opposition Liberal Democratic Party that signed off on the plan while in government.

He and other backers of the existing plan, including Ginowan businessmen, say they are concerned re-opening the issue will mean an indefinite delay to the closure of Futenma, where a 2004 helicopter crash added to fears over safety.

Gushi also sees the row as potentially undermining Japan’s U.S.-dependent security policy, leaving the country vulnerable.

“If we can’t provide the bases as we have pledged to do under the U.S.-Japan security treaty, the Americans could pull out and say they are no longer responsible,” Gushi said.

Hatoyama has said he needs time to review the existing base plan, but his Defense Minister Toshimi Kitazawa has more or less endorsed the current agreement.