US President Barack Obama has called for "meaningful action" in the wake of a shooting at a Connecticut school that left 27 dead, including 20 children.
An emotional Mr Obama spoke of his "overwhelming grief" over the deaths at Newtown's Sandy Hook Elementary School.
The suspected gunman was widely identified as Adam Lanza, 20, who is among the dead. He is also thought to have killed his mother earlier.
It is second-worst US school shooting after the 2007 attack at Virginia Tech.
The children at Sandy Hook Elementary were aged between five and 10.
Police Lt Paul Vance said 18 children were pronounced dead at the school, and two died after being taken to hospital.
Six adults were also killed, and the gunman died at the scene, apparently after shooting himself.
He is thought to have shot dead his mother, a teacher, at a house in Newtown, before heading to the school on Friday morning. Investigators say it is unclear whether she worked there.
'Broken hearts'
As Mr Obama reacted with a televised statement, he paused several times to wipe tears from his eyes.
"As a country we have been through this too many times," he said.
"We're going to have to come together and take meaningful action to prevent more tragedies like this, regardless of the politics."
The president offered condolences to the families of both the victims and survivors.
"Our hearts are broken today, for the parents, grandparents, sisters and brothers of these children, and for the families of the adults who were lost," he said.
Friday's shooting is the third major gun attack in the US in 2012.
In July an attacker killed 12 people at a premiere of a Batman film in Aurora, Colorado. In August six people died at a Sikh temple in Wisconsin.
In Washington, protesters gathered outside the White House to call for gun controls.
New York Mayor Michael Bloomberg, also called for action.
"We have heard all the rhetoric before," he said.
"What we have not seen is leadership - not from the White House and not from Congress," he said. "That must end today."
American flags on Washington's Capitol Hill were lowered to half-mast and about 200 people held a candlelight vigil for the victims.
A vigil and a memorial service were held in Newtown, a prosperous town of about 27,000 people.
'Never imagined'
Friday's killings took place in two rooms within a single section of the school, police have said. They reportedly lasted just a few minutes.
Witnesses said they heard scores of shots fired. They said teachers tried to protect children in other parts of the building by locking doors and ushering them into closets.
Firefighters reportedly told children to close their eyes and run past the school's office as they left the building.
Among the reported victims was the head teacher of the school, which was said to be highly rated.
"I've never imagined a thing like that could happen here," said David Connors, whose three children were at the school during the shooting and were unharmed.
Another parent said residents thought of Newtown as "the safest place in America".
Dressed in combat clothes, the gunman is thought to have been carrying at least two handguns.
A rifle was found in a vehicle outside the school. Three other guns were also recovered.
Officials said the suspected gunman's mother had bought several weapons legally.
Early reports named 24-year-old Ryan Lanza, of Hoboken, New Jersey, as the gunman, but unnamed officials later said his brother Adam was the suspect.
Ryan Lanza was questioned by police, US media reported, but has not been named as a suspect.
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