The Australian host city of Sydney is bracing for the arrival of world leaders and thousands of demonstrators for the Asia-Pacific Economic Co-operation summit amid unprecedented security measures.

U.S. President George W. Bush, Chinese President Hu Jintao and Canadian Prime Minister Stephen Harper are slated to join leaders from 20 APEC countries at the city's famed Opera House along to discuss issues such as trade, North Korea, Iran and global warming.

About 200 people gathered in Sydney on Tuesday to express their opposition to U.S. President George W. Bush and the Iraq war hours before Bush was to arrive for the annual APEC meeting. (AP Photo/Ed Wray)

Some 3,500 police and 1,500 counter-terrorism and special forces have been deployed to protect delegates during the weeklong summit. 

A three-metre high security fence that spans five kilometres has been erected around the summit areas as demonstrators began converging on the city on Tuesday. More than 20,000 protesters are expected during the height of talks over the weekend, according to police estimates.

The city's residents were advised to avoid the downtown area as Bush arrived late Tuesday following a surprise visit to Iraq's Sunni-dominated Anbar province.

Bush's Iraq visit — his third — came ahead of a highly anticipated progress report expected to be delivered to the U.S. congress next week that may shape the future direction of U.S. military involvement in the embattled country.

Bush begins summit talks Wednesday, meeting with Australian Prime Minister John Howard, who along with then-British Prime Minister Tony Blair was a staunch supporter of Bush in the U.S.-led invasion of Iraq in 2003.

Bush is also scheduled to meet with China's Hu, as well as leaders from Japan, China, Russia and South Korea. Some have dubbed this year's APEC meeting the "China summit," a reference to Beijing's rising influence.

"Is this a China summit? The answer is absolutely not," Bush told reporters on Air Force One during the flight to Australia. 

Before Bush's arrival Tuesday, Pacific Rim nations began negotiating a contested declaration on climate change warning that future economic well-being depends on curbing global warming. 

Australia was trying to rally support for the declaration.

Harper is expected to arrive on the weekend and will end the weeklong trip on Sept. 11 with an address to the Australian Parliament in Canberra, making him the first Canadian prime minister to do so.

With files from the Associated Press