The leaders of Australia and Japan, both staunch allies of the US, George Bush and the "War on terror", were among the first to honour victims on Monday.
The Australian prime minister, John Howard called September 11 "an attack on the values that the entire world holds in common" and promised that the ideals of liberty and freedom of religion and speech "will in the end triumph."
At a ceremony at the US embassy in Canberra, Howard defended his decision to send troops to Iraq in support of the US-led coalition, and called the campaign to remove the Taliban regime in Afghanistan the first battle in the war on terror.
"As far as Afghanistan, it's been established beyond any real argument that that was the source of the Osama bin Laden-inspired attack and the Taliban, and role of the Taliban and bin Laden in Afghanistan was really, in a sense, the beginning of this campaign of terror in the west," Howard said.
Another memorial ceremony was planned later in the day at the US Embassy in Tokyo.
Japan has also been a top supporter of the US-led war on terror, sending troops to Iraq and aiding in logistics support for operations in Afghanistan.
The Japanese prime minister, Junichiro Koizumi, in Finland for an Asia-Europe Meeting, was cited by a Japanese official in the Kyodo News agency as saying that terrorism "continued to be as much of a threat as ever to mankind."
"There's still an enemy out there that would like to inflict the same kind of damage again"
Bush himself had begun the remembrance services on Sunday when he and his wife, Laura, laid wreaths in the reflective pools where the twin towers of the World Trade Centre stood.
"There's still an enemy out there that would like to inflict the same kind of damage again," he said after the ceremony.
Americans will mark the fifth anniversary on Monday with sombre ceremonies across the country and at the sites of the devastation in New York, Washington and Pennsylvania.
Bush will meet firefighters in New York, visit Shanksville, Pennsylvania, where United Flight 93 crashed after passengers fought the hijackers, and attend ceremonies at the Pentagon.
About 3,000 people were killed by the attacks, and the anniversary has intensified the debate over whether the US, bogged down in a vicious conflict in Iraq, is any safer.
Bush's approval ratings have plummeted and he intends to use the anniversary to renew his pledge to pursue the war on terrorism.