A single white flower is left on one of the panels containing the names of the victims of the attacks on the first day that the 9/11 Memorial was opened to the public at the World Trade Center site in New York, September 12, 2011. [Photo/Agencies]

NEW YORK - Some people wept. Some embraced. Others silently stared into the dark pools where the twin towers once stood as the 9/11 memorial at ground zero opened its gates to the public.

About 7,000 people had tickets to visit the memorial as it debuted on Monday, and another 400,000 have signed up online to visit in the coming months.

Visitors said the monument, with its two 30-foot-deep voids, bronze plates with victims' names and grove of oak trees, was a stirring tribute to the dead.

"I'm actually still shaking," said Jim Drzewiecki of Lancaster, N.Y. "It could have been me on that flight. On any one of the flights. ... There's not much that separates us."

Many visitors made pencil-and-paper rubbings of the names to take back home. Others sat on benches or clustered for photos. Some left flowers or written messages.

Security was airport-tight, with visitors forced to empty their pockets, go through a metal detector and send their bags through an X-ray machine.

The memorial takes visitors through a grove of more than 200 young oak trees and to the edge of two 30-foot-deep pits where the World Trade Center's towers stood.

Water cascades into a pool at the bottom of each pit, then disappears into a second, dark void in the middle. The sight evokes eternity, said visitor Barbara Warford of Williamsburg, Va.

"In the middle where you can't see the bottom, it's never-ending," Warford said. "Water flows and it just goes forever."

The bronze plates carry the names of the 2,977 people killed in the terrorist attacks in New York, at the Pentagon and in Pennsylvania, plus the names of the six who died in the bombing of the trade center in 1993. The letters have been cut all the way through the metal, with empty space beneath them.

Nearby are a half-dozen electronic directories to help visitors find names, which are grouped not alphabetically but in ways that showed the connections between co-workers, firefighters, airplane flight crews and other victims.

Visitors said they found it profoundly moving.

"I feel like an ice cube," Marijke Bos, 60, of Rotterdam, Netherlands, said after visiting the memorial. "I still have just chills."

The memorial's architect, Michael Arad, said the plaza next to the pools was inspired by gatherings of mourners that he saw in New York's Washington Square and Union Square after the attacks.

"These places don't just bring us together physically in one spot, they brought us together emotionally," Arad said. "We've recreated that opportunity for that to happen here."

There is a separate entrance for 9/11 family members and comrades of the fallen firefighters and police officers. Certain days or hours will be set aside for them to visit privately.

Workers are still building the 9/11 museum underneath the memorial. It is scheduled to open in 2012 and will include two of the forklike supports that were left standing when the World Trade Center fell, as well as a stairway that enabled hundreds to escape.

Construction also continues next door on 1 World Trade Center, which is more than 80 stories high so far and will be the nation's tallest building at 1,776 feet.

It is one of several new buildings that will eventually surround the memorial. Two World Trade Center will be 1,349 feet high with a diamond-shaped tip and an 80-foot antenna. The 53 stories of 3 World Trade Center will feature crisscross external braces.

Admission to the memorial is free, but visitors must obtain passes in advance that allow them to enter at a specified time.

The cost of the memorial and museum has been put at about $700 million. The nonprofit organization that runs the project has raised about $400 million in private donations and is seeking federal funds.