In Washington, President George Bush observed the first moment of silence on the South Lawn of the White House after attending a special church service on Thursday.
Bush said: "Today, our nation remembers: we remember a sad and terrible day … we remember lives lost. We remember the heroic deeds. We remember the compassion and the decency of our fellow citizens on that terrible day."
In London, Princess Anne opened a commemorative garden in London in a low-key ceremony to which relatives of all Britons killed in the twin towers were invited.
Spanish Prime Minister Jose Maria Aznar expressed solidarity with the US, and described terrorism as “a threat which we must combat with force, determination and the collaboration of all nations."
France, meanwhile, highlighted the continuing instability, with Defense Minister Michele Alliot-Marie saying France remained a target, albeit a secondary one.
A day that will never be forgotten
"Terrorism is one of the constant concerns" of the government, Alliot-Marie told LCI television. "We are not the first ones in their sights, but we are potential targets," she said.
In Manila, President Gloria Arroyo warned that the second anniversary "reminds us that we should not take the threat of terrorism lightly, for we know that there are still a number of terror groups waiting for a chance to strike."
Prime Minister John Howard of Australia said the War on Terror "could go on for a very long time, just as the Cold War went on for a long time.
"It is just a new reality that the world has to come to grips with. I don't like it. None of us like it. It's frightening. It's dreadful," he said.
Newspapers in Lebanon and Jordan, meanwhile, accused the US of exploiting the attacks to impose its dominance over the Middle East.
The al-Nahar newspaper said that two years after the strikes, the War on Terror was gathering momentum.
After invading Afghanistan and
Iraq, US no closer to capturing
"The United States plunged into a vicious circle, and far from being capable of directing this war and ensuring good results, it has become a hostage to it," the paper said.
Citizens expect more
Americans are still shocked by the strikes that forced them to acknowledge their vulnerability and face up to the hostility their government’s foreign policy causes in many parts of the world.
Though the country has not been hit with additional domestic attacks since then, most appear resigned to the idea that terrorists will strike again, sooner or later.
"Each of you now has a part in protecting America against the threats of the new era," Bush said.
Surveys show most Americans polled still expect more strikes.
A recent University of Maryland poll of 1217 people showed that between 70 and 80% of those surveyed fear further attacks.
Twenty-eight percent of those surveyed said they felt less safe than they did two years ago, while 48% said they were a bit safer. Only 24% believed they were safer.