In England, the Prince of Wales and Prince Harry led the memorial service at St Paul's Cathedral.
They were joined by the country's political leaders and the families of some of the 67 British victims.
During the service thousands of white rose petals were dropped from the roof of the cathedral to mark each of the lives lost on 9/11.
In Australia, more than 3,000 people in red, white and blue tee-shirts turned out on a beach near Brisbane to make a giant American flag.