Officials didn't make any formal remark, and similar commemorations took place throughout the country.
Though the massacre does not rank as the deadliest school shooting in U.S. history - that happened at Virginia Tech - the tender age of the victims and the absence of any apparent motive has struck at Americans' hearts and minds. The gunman used a military-style assault rifle loaded with ammunition intended to inflict maximum damage, officials have said.
The White House said President Barack Obama privately observed the moment of silence.
Just a week after the attack, gun control has taken a front burner in Congress, where previous mass shootings produced only minimal legislative reaction. Vice President Joe Biden said Thursday that the Obama administration would push to tighten gun laws.
The National Rifle Association, at its first public event since the shootings, called Friday for armed police officers to be posted in American school to stop the next killer "waiting in the wings."
Wayne LaPierre, CEO of the nation's largest gun-rights lobby with 4.3 million members, said at the Washington news conference that, "The only thing that stops a bad guy with a gun is a good guy with a gun."
He blamed video games, movies and music videos for exposing children to a violent culture.
Though security was tight, the briefing was interrupted twice by people holding up signs that blamed the NRA for killing children. The protesters were taken from the room.
Traffic stopped in the streets outside the town hall in Newtown as bells rang out to honor the dead.
Malloy, taking deep breaths with his hands folded in front of him, was joined by the Newtown superintendent of schools, lawmakers and other officials as bells rang out at the nearby Trinity Episcopal Church.
Firefighters bowed their heads around a memorial filled with teddy bears, other stuffed animals and a New York Giants pillow. Some hugged and onlookers shook their hands afterward.
"When I heard the 26 bells ring it just melted my soul," said Kerrie Glassman, of Sandy Hook, who said she knew seven of the victims. "It's just overwhelming. You just can't believe this happened in our town."
Among those who gathered in Newtown was a group of 13 survivors of the 2005 school shooting on the Red Lake Indian Reservation in Minnesota. The group drove nearly 1,500 miles to support and comfort the families and survivors. They brought gifts intended to bring a message of resilience and hope, including a plaque that survivors of the 1999 Columbine shooting gave to them after their experience.
"This is just something we had to do," said Ashley Lejeunesse, 23, who was also in the Red Lake classroom.
The chiming of bells reverberated throughout the nation and there were observances around the world.
In Washington, religious leaders from a broad range of faiths gathered at Washington National Cathedral to call for their congregations to lobby Congress to enact gun control and mental health reforms to address pervasive gun violence. In a garden beside the National Cathedral, they paused to listen as a funeral bell tolled.
In New York City, bells at the historic Trinity Church near the World Trade Center tolled 28 times. In Massachusetts, bells in churches around the state, including Boston's historic Old North Church, rang in honor of those killed in the attack. A moment of silence was observed throughout Colorado, and bells rang out in Denver.
In the west African nation of Liberia, 20 children from a school sponsored by the Newtown Rotary Club gathered at the U.S. Embassy to give their condolences. Each child from the Caroline Miller School in Monrovia placed a flower on a poster bearing the name of a victim of the shooting.
When the bells tolled to honor the victims of last week's shooting rampage, they did so 26 times, for each child and staff member killed.
There is rarely a mention by residents of the first person police said Adam Lanza killed that morning: his mother, Nancy, who was shot in the head four times while she lay in bed.
A private funeral was held Thursday in New Hampshire for Nancy Lanza, according to the police chief in Kingston, N.H., where her funeral was held. About 25 family members attended.
The Newtown area weathered another day of funerals Friday, with five planned.
A standing room-only crowd filled the St. Stephen Roman Catholic Church in Trumbull for the funeral of Mary Sherlach. The school psychologist who rushed toward the gunman during the shooting was remembered as a caring professional, a fan of the Miami Dolphins and a woman who ultimately put the lives of others ahead of her own.
Investigators have said that Nancy Lanza, a gun enthusiast, visited shooting ranges several times and that her son also visited an area range.
Authorities say Adam Lanza shot his mother at their home and then took her car and some of her guns to the school, where he broke in and opened fire. A Connecticut official said Nancy Lanza was shot four times in the head with a .22-caliber rifle.
Adam Lanza was wearing all black, with an olive-drab utility vest, during the school attack. Investigators have found no letters or diaries that could explain the rampage.
Friends and acquaintances have described him as intelligent, but odd and quiet.
Friends said he would stare down at the floor and not speak when she brought him into a local pizzeria. They knew that he'd switched schools more than once and that she'd tried home schooling him. But while she occasionally expressed concern about his future during evenings at the bar, she never complained.
Contributing to this report were Associated Press writers Michael Melia, John Christoffersen, Eileen Connelly, Susan Haigh and David Klepper in Newtown.