He vowed to use all his power to make sure that shooting tragedies like the one that left 20 small children and six adults dead in Newtown are not repeated.
Obama was meeting victims' families and first responders who had to view Friday's horrific carnage at Sandy Hook Elementary School in Newtown, a quaint postcard-pretty town of 27,000 haunted by the most appalling crime.
"We can't accept events like this as routine," Obama told a poignant multi-faith vigil in the Connecticut town. "We as a nation are left with some hard questions. These tragedies have to end, and to end them we must change."
An impassioned Obama offered the "love and prayers of a nation" to families of the victims, saying all Americans stood by their side in mourning the tragic loss.
"I can only hope it helps for you to know that you're not alone in your grief; that our world, too, has been torn apart; that all across this land of ours, we have wept with you," Obama said.
"We've pulled our children tight, and you must know that whatever measure of comfort we can provide, we will provide. Whatever portion of sadness that we can share with you to ease this heavy load, we will gladly bear it.
"Newtown, you are not alone."
Officials formally identified Adam Lanza, 20, as the shooter and confirmed that he shot his mother several times in the head at the house they shared before going to his old school and embarking on a gruesome killing spree.
Despite the rain and cold conditions, queues stretched for at least 200 yards (meters) outside the auditorium at Newtown High School, where Obama was to attend a vigil later on Sunday evening and give an address.
Adults stood in groups, some crying and hugging, others joining younger children, many of elementary school age, in carrying teddies and cuddly toys as symbols of remembrance for young lives tragically taken too soon.
Lanza used his mother's Bushmaster .223 assault rifle to kill 26 people at the school, including 20 children aged either six or seven, before taking his own life with a handgun as police officers closed in and sirens wailed.
During his rampage, the shooter had four guns and multiple magazines, some holding up to 30 clips, but Connecticut State Police spokesman Lieutenant Paul Vance said it was unclear how many bullets were fired.
Connecticut's chief medical examiner Wayne Carver has said that the bodies of the child victims -- all aged just six or seven -- were riddled with as many as 11 bullets.
Vance declined to hint at any possible motive they may have uncovered so far in their investigation, saying: "We don't have a specific reason we can stand here and say this occurred."
As Obama visited heartbroken Newtown, there was an increasing clamor from gun control activists to use the tragedy as a tipping point to finally force action to tackle the nation's lax laws on firearm ownership.
A prominent Democratic lawmaker, senator Dianne Feinstein of California, promised to introduce a bill to ban assault weapons on the very first day of the next Congress, January 3.
In ways big and small, tributes were paid -- from candles lit and teddy bears left at the elementary school crime scene, to gestures at the cavernous football stadiums that usually fixate Americans' attention on Sundays.
Before the day's games around the country, the National Football League had teams observe a minute's silence in memory of those killed.
Back in Newtown, nerves remained on edge. One Catholic church where people attended services -- Saint Rose of Lima -- was evacuated due to an undisclosed threat. Armed police searched a house next door.
Townsfolk poured into churches to pray and seek solace over the unimaginable -- a gunman pumping shot after shot into small children with a rifle of the kind used in wars.
The town Christmas tree became an impromptu place of remembrance, with people pausing every few minutes to pray and cross themselves under a light snowfall.
One middle-aged woman knelt down in front of the ranks of votive candles, teddy bears and handwritten notes, and bowed her head in tears.
"The community is gathering together and praying," Red Cross volunteer Rosty Slabicky told AFP.
"They are destroyed... Not just the families, but the first responders are dealing with the crisis on a very personal and emotional level."
The investigation entered a new stage with the autopsy of Lanza, seen as a withdrawn and awkward youngster who had shown no signs of violence, let alone any indications that he might perpetrate a massacre.
Lanza's main weapon was the Bushmaster, a civilian version of the US military's M4 -- legally registered to his mother. Police said he had three other weapons with him, two pistols and a shotgun found in a car.
Connecticut governor Dannel Malloy revealed that Lanza blasted his way into the school, which had just installed a new security door where visitors could be viewed by video camera and buzzed in.
"He shot his way into the building. He penetrated the building by literally shooting an entrance into the building. That's what an assault weapon can do for you," Malloy said on CNN.
Many states, including Connecticut, already have strict laws on the purchase of firearms, but with no federal statutes, there is little to stop the traffic of guns from other states where fewer restrictions apply.
An assault weapon ban was passed in 1994 under president Bill Clinton but it expired in 2004 and was never resurrected. Obama supported restoring the law while running for president in 2008 but did not make it a priority during his first term.
"We have to come together and take meaningful action to prevent more tragedies like this," Obama said in his weekly radio address on Saturday. "Regardless of the politics."
However, with gun ownership protected by the constitution and firearms popular among a broad base of Americans, especially conservative Republicans, gun bans have long been seen as a vote-losing proposition.
Mass shootings are all-too-frequent in the United States. Only last Tuesday, a man with a semi-automatic rifle raked an Oregon shopping mall, killing two people then taking his own life.
Newtown was the second deadliest school shooting in US history after the Virginia Tech massacre in 2007 in which South Korean student Seung-Hui Cho shot and killed 32 people and wounded 17 others before taking his own life.
In the most notorious recent incident, a 24-year-old, James Holmes, allegedly killed 12 people and wounded 58 others when he opened fire at a midnight screening of the latest Batman movie in Aurora, Colorado, in July.