New York ? Silence fell at ground zero Monday morning, six months after hijacked jetliners obliterated two of the world’s tallest buildings. The thousands who died that day “would want us to lift up our heads very, very high,” said the city’s former mayor.

Twice at 8:46 a.m. and 9:03 a.m. mourners near the ruins of the World Trade Center observed moments of silence, marking the precise times when the planes crashed into the towers.

“The time between the first and second plane is just a brief moment in our lives,” Mayor Michael Bloomberg told a crowd of several hundred who gathered at Battery Park, just blocks from ground zero. “We have to go on and we will go on … We cannot let our guard down ever again. We must remember.”

Bloomberg asked them to “look into your hearts to remember those that are no longer with us and also to think about how we can go forward, never forget those that we lost, but also to build the kind of future that they will want for all of us.”

His predecessor, Rudolph Giuliani, urged them to the victims “for our inspiration and our sense of purpose. They would want us to lift up our heads very, very high” and move forward.

“Life means all that it ever meant,” said Peter Raimondi, 12, reading a poem with his brother, Philip, 16. The boys lost their father, Peter.

At the same time, President Bush held a solemn ceremony on the White House South Lawn, joined by more than 100 ambassadors, as well as relatives of some victims and members of Congress.

“There can be no peace in the world where differences and grievances become an excuse to target the innocent for murder,” Bush said. “Against such an enemy, there is no immunity, and there can be no neutrality.”

The president said Sept. 11 was a day when the world “was stirred to anger and to action” over terrorism. “And the terrorists will remember Sept. 11 as the day their reckoning began,” he said.

And in Shanksville, Pa., church bells began to toll at 10:06 a.m. in honor of each victim aboard Flight 93, the fourth of the airliners hijacked on Sept. 11. Calls from people on Flight 93 to loved ones suggested the passengers confronted the hijackers before the plane crashed in a field outside the western Pennsylvania town.

Representatives of several denominations spoke during the ceremony, including a Muslim cleric.

“The first thing hijacked was our name and our religion. We cannot condemn a nation or a religion for a single act, a criminal act against humanity,” said Imam Fouad El Bayly, from the Islamic Center of Johnstown, Pa.

At Battery Park, Giuliani said he wondered during the first moments of the attack whether the city could endure and get through the tragedy.

“Shortly after, during the first day, I realized that your loved ones gave us the example on which we would build,” Giuliani said.

Gov. George Pataki said “we saw the face of evil” on Sept. 11.

“And yet Sept. 11 will also stand for our response, which was to respond to evil with good, to respond to terror with love,” he added.

At police precincts citywide, the names of the 23 officers killed were read aloud at 8:30 a.m.

“They were called on to act and did so with the highest valor,” Capt. David Barrere said as he read the names outside the 76th Precinct in Brooklyn, as two dozen officers lined up on the sidewalk.

During the Battery Park ceremony, city officials also dedicated a sculpture damaged in the Sept. 11 attack as a temporary memorial to the victims.

“The Sphere,” a steel and bronze sculpture that stood in the fountain of the trade center plaza, was gashed and partially crushed by falling debris. It was created in 1971 by artist Fritz Koenig and was dedicated as a monument to world peace through international trade.

“The sphere may be damaged but our belief in the principles it represents has never been stronger,” Bloomberg said. “The real memorial will be in our hearts,” he said.

Bloomberg said the globe probably would serve as a centerpiece for a permanent memorial.

Lucy Hindle, whose niece, Lucy Fishman, 36, died in the south tower, was among those in the crowd. “It was very emotional because we haven’t found her yet,” Hindle said tearfully. “It seems like the longer it gets, the harder it gets.”

Edwin Morales, lost his cousin, Ruben Correa, a firefighter from Engine 74.

“I know some people say this is too hard, but this is something I need,” Morales said. “I need to be here.”

At St. Paul’s Chapel near ground zero, the names of the attack victims from the trade center, the Pentagon and the plane crash in Pennsylvania were read aloud. The church was a relief center during the months after the attack, and still serves breakfast to recovery workers digging through the rubble.

At the U.S. Embassy in Kabul, Afghanistan, the flag flew at half-staff Monday.

Embassy personnel gathered at the flag at the end of the day while U.S. Marine guards held a simple ceremony marking the day that prompted the war there. The flag was lowered as a bugle sounded in the background.

In New York, the day of remembrance was to conclude with the ceremonial illumination of the “Tribute in Light” on a vacant lot next to the trade center complex. It will consist of two searchlights sending 88 high-powered beams of light into the night sky.

The light towers, created by two arts organizations, will be displayed until April 13.