Bagpipers led solemn marches from far-flung neighborhoods to ground zero in lower Manhattan early Wednesday to begin a daylong series of events. New York City's official memorial service got under way with a moment of silence at 8:46 a.m., the exact time the first hijacked jet crashed into the World Trade Center.
"Again today we are a nation that mourns. Again today we take into our hearts and minds those who perished on this site one year ago," said Mayor Michael Bloomberg.
On the eve of the anniversary, the Bush administration raised the nationwide terror alert to its second-highest level, heightened security at federal buildings and landmarks, and closed nine U.S. embassies overseas as new intelligence warned of car bombings, suicide attacks and other strikes.
Pentagon officials said Wednesday that the U.S. military command in charge of the region from East Africa through the Arabian Peninsula to Pakistan raised its security level overnight.
At home, the U.S. military expanded jet fighter patrols over some 10 cities and armed missile launchers that had already been stationed near several Washington sites, defense officials said.
Despite the threats, federal officials told Americans to go ahead with their plans to commemorate Sept. 11 at gatherings and not change their work, school or travel schedules.
At ground zero, former New York Mayor Rudolph Giuliani led a parade of dignitaries reading the names of the 2,801 people killed or missing in the trade center attack. Name readers included relatives of victims from hard-hit companies like Cantor Fitzgerald and Aon. Secretary of State Colin Powell, Sen. Hillary Rodham Clinton and actor Robert De Niro also were on the list.
The first name read was that of Gordon Aamoth Jr., 32, who worked for investment firm Sandler O'Neill & Partners on the south tower's 104th floor. The roll ended with Igor Zukelman, 29, who worked at Fiduciary Trust Company International in the north tower.
"They were our neighbors, our husbands, our children, our sisters, our brothers and our wives. They were our countrymen and our friends. They were us," Bloomberg said.
At 10:28 a.m., the time the second tower collapsed, a second moment of silence was observed.
President Bush will lay a wreath at ground zero in the afternoon, and at 9:01 p.m., he is to deliver an address to the nation from Ellis Island, with the Statue of Liberty as his backdrop.
Mr. Bush also attended a ceremony Wednesday morning at the Pentagon, where 189 people were killed on Sept. 11 when American Flight 77 crashed into the building. The president and Defense Secretary Donald Rumsfeld stood beneath a massive American flag as it was unfurled on the side of the restored building.
"We renew our commitment to win the war" against terrorism, Mr. Bush declared.
"The murder of innocents cannot be explained, only endured," a somber president said. "Though they died in tragedy, they did not die in vain," Mr. Bush told a crowd of several thousand.
The president traveled later to Shanksville, Pa., to take part in another solemn ceremony in honor of the passengers and crew of United Flight 93, who have been hailed as heroes for struggling to take back their hijacked plane from four terrorists.
A bell tolled for each of the 40 victims, accompanied by the reading of each of their names, leading up to the moment that the plane crashed at 10:06 a.m. last Sept. 11.
Homeland Security Director Tom Ridge, who was Pennsylvania's governor at the time of the attacks, said the nation owed its gratitude to the passengers who fought back, and compared them to fallen soldiers.
"Your loved ones did not expect to serve the cause of freedom on that Tuesday morning. But serve it, they did. Faced with the most frightening circumstances one could imagine, they met the challenge like citizen-soldiers, like Americans."
The day was being marked in smaller ways in thousands of American cities and towns that felt the shockwaves of the worst terrorist attacks in U.S. history.
The strains of Mozart's Requiem were heard in every time zone, sung by symphonies and school choirs.
Splinters of the destroyed buildings were on display in states such as Nevada, Tennessee, Ohio and Wyoming.
Public schools in several states honored the day with special events, including a moment of silence at 9:40 a.m. in all Washington, D.C. classrooms. That's the moment when a plane carrying three Washington students and three teachers hit the Pentagon.
On the sprawling statehouse lawn in Columbus, Ohio, 2,999 American flags and one Ohio flag were arranged to depict the twin towers. In San Francisco's Washington Square, more than 3,000 flags flew, including those of 14 other countries whose citizens were among the victims.
At Boston's Logan International Airport, where the two planes that struck the trade center took off, all ground operations stopped at 8:46 a.m.
Attesting to the global impact of Sept. 11, gatherings to mark the anniversary were also taking place around the world. Nearly 500 foreigners from 91 countries lost their lives in the terror attacks.
In Britain, which lost 67 citizens, more than any country other than the United States, a service of remembrance and commemoration was held at London's St. Paul's Cathedral.
In Japan, which lost 24 citizens, flowers were left in front of the U.S. embassy, and a group of Buddhist monks gathered in the same place, to chant memorial prayers.
At the Vatican, Pope John Paul II said Wednesday that nothing can justify "such an aberration" as the Sept. 11 attacks, and urged rich countries to do all they can to put an end to "scandalous" injustices.
"Terrorism is and always will be a show of inhuman ferocity," the pope told some 8,000 pilgrims packing a Vatican auditorium. "It will never be able to solve conflicts among human beings."
Other memorials were taking place in Australia, New Zealand, Israel, Germany, Denmark, Finland and France, where President Jacques Chirac told a ceremony at the heavily guarded U.S. Embassy in Paris that "France knows what it owes America."
"The French people stand with all their hearts at the side of the American people," Chirac said at the gathering Wednesday with U.S. Ambassador Howard H. Leach and 300 guests.
At the U.S. Navy Base at Guantanamo Bay, Cuba, Marines held their own small ceremony. At the prison camp nearby, it was just another day behind bars for the 598 men from 43 nations suspected of links to al Qaeda or the Taliban. The detainees have no calendars and were not being told it was Sept. 11.
By JOEL ROBERTS