President Obama has urged "meaningful action" against gun crime in the US after a school shooting in Connecticut 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, widely identified as 20-year-old Adam Lanza, is among the dead. Before going to the school he had also killed his mother at their home.
It is the second deadliest shooting attack at a US school or university.
In 2007, a student at Virginia Tech university killed 32 people and injured many more.
The children at Sandy Hook Elementary were aged between five and 10.
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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 - including the school's principal Dawn Hochsprung - and the gunman died at the scene, apparently after shooting himself.
His mother was found dead at their home on Yogananda Street - some unconfirmed reports suggested she had worked at the school.
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.
The deaths have sparked fresh debate over the country's gun laws.
In Washington on Friday night, protesters gathered outside the White House to call for tighter 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. 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.
Friday's killings took place in two rooms within a single section of the school, police have said. The shooting lasted just a few minutes.
As they heard the shots, teachers in other parts of the building tried to protect children by locking doors and ushering them into closets.
"I told them we had to be absolutely quiet, because I was just so afraid if he did come in, then he would hear us and just start shooting the door," said teacher Kaitlin Roig.
"I said to them, 'I need you to know that I love you all very much and that it's going to be okay', because I thought that was the last thing they were ever going to hear."
Firefighters who arrived to evacuate the school reportedly told children to close their eyes and run past the school's office as they left.
The gunman was dressed in combat clothes and 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 his mother had bought several weapons legally.
Witnesses said the gunman said nothing during the attack.
"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.
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.