NEW YORK (AFP)- The United States paused in solemn and tearful remembrance of the September 11 attacks, as President George W. Bush vowed to pursue a relentless war on terrorism.
With the country divided by a bitter presidential election campaign, the third anniversary of the attacks provided an opportunity for Americans to unite in shared grief for the nearly 3,000 people who perished.
In New York, the remembrance ceremony at Ground Zero was led by 200 parents and grandparents of those killed at the World Trade Center, as they read, in voices choked with emotion, the roll call of the dead.
"There is no name for a parent who loses a child, for there are no words to describe this pain," said New York Mayor Michael Bloomberg.
The somber commemoration was punctuated by four moments of silence, marking the times the two hijacked planes slammed into the trade center and the collapse of each of the twin towers.
Two powerful spotlights were later to send two shafts of light up into the night sky to symbolize the fallen structures.
Bush, who observed the first minute of silence on the lawn of the White House, later issued a combative warning to terrorist groups around the world.
"The United States is determined to stay on the offensive and to pursue the terrorists wherever they train or sleep or attempt to set down roots," he said in a live radio address from the Oval Office, where he was joined by victims` relatives.
"We know that there is still a danger to America, so we will not relent until the terrorists who plot murder against our people are found and dealt with," he added.
For Bush, September 11 remains the defining moment of his presidency and the foundation of his national security policy, which has become one of the key areas of debate in the run-up to November`s election.
At the national Arlington Cemetery, Defence Secretary Donald Rumsfeld laid a wreath for the 184 people who died in the attack on the Pentagon, and spoke of September 11 as a "call to arms."
"The lives that were lost ... have meaning," he said. "They live on as a testament to a country that is courageous, that is determined, to a people that are resilient despite great loss and to a cause that continues until that mission is accomplished and beyond."
For Americans, the human cost of the war on terror was underlined just days before the third anniversary, when the death toll among US soldiers in Iraq crossed the 1,000 mark.
At the Ground Zero ceremony, thousands of victims` relatives held portraits of their lost loved ones, with messages that ranged from expressions or remembrance and grief to those of anger and revenge.
"Still waiting for justice" read one placard.
Many carried roses down into the pit of the trade center site and placed them in two square pools of water -- a scaled-down version of what will become part of a permanent memorial to those who died.
The third anniversary marked perhaps the last time the families will be able to descend to the bedrock of Ground Zero, which by next year will be transformed by reconstruction work.
Meanwhile, in Pennsylvania, bells pealed across the state in memory of the 44 people who died when a fourth airliner crashed in a field after passengers staged a rebellion against the hijackers.
The prospect of a fresh terrorist attack remains a potent fear for many Americans, who received a taunting reminder on Thursday of Al-Qaeda`s hostile intentions.
In a video broadcast, Osama bin Laden`s deputy, Ayman al-Zawahiri, said Americans would never be safe while their government "commits crimes against Muslims in Iraq and Afghanistan and Palestine."
The US State Department Friday urged US citizens abroad to be vigilant, warning that Al-Qaeda and affiliated groups were preparing to strike US interests overseas.
Bush`s presidential challenger, Democratic Senator John Kerry, attended a commemorative event at the opera house in his hometown of Boston, from where some of the hijacked planes used in the September 11 attacks took off.
"We will always remember where we were on that day," Kerry said. "And we must always remember that we will only defeat those who sought to destroy us by standing together as one America."
Despite the somber atmosphere in New York, there were also clear signs that the city and its inhabitants are increasingly looking to the future.
The gaping scar that was Ground Zero is already a burgeoning construction site, where a groundbreaking ceremony was held July 4, Independence Day, for the symbolic 1,776-foot (540-meter) Freedom Tower that will form the centrepiece of the redeveloped site.
The city`s economy is back on track after three harsh years, with tourist numbers back up and small businesses returning in force to the lower Manhattan area.
In Baghdad, US soldiers gathered in an opulent palace built by Saddam Hussein for a memorial ceremony to those killed in the September 11 attacks and the 1,100 coalition soldiers killed since the US-led invasion of Iraq last year..
Calling September 11 "an attack on the universal ideals of democracy," General George Casey said the subsequent US-led war on terror had freed 50 million people "from the grip of terrorists and tyrants around the world.
"When we win the war in Iraq it will be the beginning of the end of the war on terror."