The memorial plaza opened its gates under tight, airport-style security. There are now two giant fountains and reflecting pools where the World Trade Center's twin towers once stood.
Admission to the memorial is free, but access will be tightly controlled. Visitors need to obtain passes in advance, allowing them to enter at a specified time. No more than about 1,500 at a time will be allowed in.
The museum portion of the memorial complex is still under construction. The museum pavilion, a tilting structure that evokes the sections of the trade center facade that remained standing after the towers fell, is scheduled to open on the 11th anniversary of the attacks.
Ceremonies honoring the victims were held across the nation Sunday. President Barack Obama and former President George W. Bush, along with their wives, walked along the memorial pools as part of the commemoration.
Families spoke out, remembering lost loved ones. The names of the 2,977 people killed in the attacks in New York, at the Pentagon and in Pennsylvania, along with the six killed in the 1993 World Trade Center bombing, are engraved in bronze around the fountains.
Tom Frost, whose daughter was on United Flight 175 that crashed into the South Tower, was in New York City for the somber anniversary.
"I see these fountains down here, and it's beautiful," said Frost. "I know that this is a fitting tribute to all those who died that day."
Obama visited all three sites, ending at the Pentagon in Washington, D.C. Wreaths were placed at a memorial where each of the 184 victims is remembered with a bench and small reflecting pool.
The president met with about 100 people who lost loved ones in the attacks.
Earlier, he and the first lady met with families of those killed in Iraq and Afghanistan at the Arlington National Cemetery.
Obama also visited the crash site in Shanksville, Penn., where United Airlines Flight 93 crashed after passengers and crew members fought the hijackers for control of the plane.
Remains recovered from the crash will be interred, and a private funeral will be held for some family members in the new Flight 93 National Memorial.
Locally, ceremonies marking the anniversary of the Sept. 11 attacks were held in several communities.
An event was held at Fire Station 88 in Sherman Oaks, where members of an elite search and rescue team are based. That team spent several days searching for survivors at Ground Zero.
Hundreds gathered near a memorial fountain at the station to remember the victims and to honor the rescuers.
More events commemorating 9/11 are planned for Monday.
Fairvalley High School in Covina plans to hold a remembrance ceremony.
Students in the leadership class at Sun Valley Middle School will observe the anniversary with a daylong multimedia museum.
The Associated Press contributed to this report.