By Rob Taylor

CANBERRA (Reuters) - Harborside Sydney, usually a tourist and restaurant mecca, will be transformed into a high-security fortress when the leaders of 21 Asia-Pacific economies gather for the three-day APEC summit.

More than 4,000 troops, police, federal agents and private security guards will be deployed in the city centre to guard leaders including U.S. President George W. Bush, Japanese Prime Minister Shinzo Abe and China's President Hu Jintao.

Sydneysiders are being asked to have a holiday during the September 7-9 meeting as the city centre is locked down within a 2.8-metre (9ft) reinforced concrete fence, meant to keep at bay possibly thousands of anti-globalization protesters.

"We are encouraging people to take a break over that long weekend, because the Sydney CBD and particularly the northern part won't be the place to be," New South Wales state Transport Minister John Watkins said on Wednesday.

The Asia-Pacific Economic Cooperation forum draws together leaders of 21 economies accounting for more than a third of the world's population, about 60 percent of global GDP and 47 percent of world trade volume.

Host Australia hopes to highlight climate change at this year's summit, with Kyoto climate pact holdout Canberra seeking to build support for a "New Kyoto" including developed economies such as the United States and emerging heavyweights China and India.

Demonstrators, including the so-called "Stop Bush Coalition", promised peaceful protests during the summit, criticizing police no-go zones and mobile detention buses in parts of the city close to the Harbour Bridge and historic Rocks tourist precinct.

Most of the major five-star hotels are in the sealed-off area.

Leaders' meetings will be held on September 8 at the recently World-Heritage-Listed Sydney Opera House, and September 9 at the nearby Government House