Americans across the country are taking part in emotional ceremonies to mark the anniversary of the September 11 terrorist attacks - honoring the more than 3,000 people who perished, as well as the heroes who tried to save them.

During a solemn ceremony at New York's Ground Zero, relatives of those killed at the World Trade Center stood for a moment of silence at exactly 8:46 a.m. local time, the moment when the first of two hijacked planes sliced into the towers.

New York Mayor Michael Bloomberg joined with relatives of the victims to mourn their loss. "Again today, we are a nation that mourns," he said. "Again today, we take into our hearts and minds those who perished on this site one year ago."

Former Mayor Rudolph Guiliani, the man who led New York through its darkest hour, began the emotional process of reading the names of the 2,800 people who died in the World Trade Center attack, which plunged the nation into its on-going war on terrorism.

Near Washington, at the newly refurbished Pentagon building and in Shanksville, Pennsylvania, the sites where two other hijacked planes crashed a year ago, mourners also observed a moment of silence to pay tribute to those who lost their lives. At the Pentagon ceremony, President Bush looked back at the events a year ago and vowed the United States will win the war against terrorism. "What happened to our nation on a September day set in motion the first great struggle of a new century," said President Bush. "The enemies who struck us are determined and they're resourceful. They will not be stopped by a sense of decency or a hint of conscience. But they will be stopped."

People across the world are also observing this anniversary, including in Afghanistan where ceremonies were held at the U.S. embassy and at bases housing American troops, who continue to provide security in the war torn country.

Amid the anniversary ceremonies, security has been a concern. President Bush ordered the nation placed on its highest state of alert since last year's attacks - with Attorney General John Ashcroft warning the most serious threats concern American government sites in Asia. "Recent events parallel terrorist activity that occurred in the weeks prior to last year's attack," he said.

A number of American embassies, most of them in Asia, were closed and Americans around the world were warned to remain especially vigilant. In Washington, Vice President Dick Cheney was taken to an undisclosed, secure location, a precaution in the event an attack would incapacitate the President. The skies over Washington and New York have been put under jet fighter patrol and Defense Secretary Donald Rumsfeld ordered anti-aircraft missile batteries deployed at the Pentagon and other locations - the first deployment of its kind across the nation's capital since the Cuban missile crisis of 1962.