A heavily armed gunman opened fire inside a Connecticut elementary school, killing 26 people, including 20 children.

The gunman - who according to one media report carried four weapons and wore a bullet-proof vest - was dead inside Sandy Hook Elementary School in Newtown

State police Lieutenant Paul Vance said authorities found 18 children and seven adults, including the gunman, dead at the school.

Two children were later pronounced dead at a hospital.

Lt Vance said another adult was found dead at a related crime scene in Newtown, bringing the toll to 28.

The New York Times reported that the gunman, believed to be in his 20s, walked into a classroom where his mother was a teacher, shot her and then 20 students in the room before shooting five other adults and killing himself.

If confirmed, it would be one of the worst mass shootings in US history.

The tragedy was the second shooting rampage in the United States this week and was certain to revive a debate about US gun laws.

Witnesses reported hearing dozens of shots with some saying as many as 100 were fired.

Another person was being held in police custody after he was detained in the woods near the school wearing camouflage pants, CBS reported.

Children aged between five and ten

Sandy Hook Elementary School teaches children from kindergarten through fourth grade - roughly ages five to ten.

"It was horrendous," said parent Brenda Lebinski, who rushed to the school where her daughter is in the third grade. "Everyone was in hysterics - parents, students.

"There were kids coming out of the school bloodied. I don't know if they were shot, but they were bloodied."

Television images showed police and ambulances at the scene, and parents rushing toward the school.

Parents were seen reuniting with their children and taking them home.

"This is going to be bad," a state official told Reuters, requesting anonymity because the scope of the tragedy remained uncertain.

Ms Lebinski said a mother who was at the school during the shooting told her a "masked man" entered the principal's office and may have shot the principal.

Ms Lebinski, who is friends with the mother who was at the school, said the principal was "severely injured."

Ms Lebinski's said her daughter's teacher "immediately locked the door to the classroom and put all the kids in the corner of the room."

She did get an automated call from the school department this morning saying there was a "possible shooting" at the elementary school and that all schools were on lockdown.

Melissa Murphy, who lives near the school, monitored events on a police scanner.

"I kept hearing them call for the mass casualty kit and scream, 'Send everybody! Send everybody!'" she said.

"It doesn't seem like it can be really happening. I feel like I'm in shock."

A girl interviewed by NBC Connecticut described hearing seven loud "booms" while she was in gym class.

Other children began crying and teachers moved the students to a nearby office, she said.

"A police officer came in and told us to run outside and so we did," the unidentified girl said.

Newtown, with a population about 27,000, is in northern Fairfield County, about 45 miles (70 km) southwest of Hartford and 80 miles (130 km) northeast of New York City.

President Obama demands action

A tearful US President Barack Obama expressed "overwhelming grief" for the victims of the shooting rampage and called on Americans to set aside politics and "take meaningful action" to prevent more tragedies of this kind.

"Our hearts are broken today," Mr Obama said, his voice breaking at times during a nationally televised appearance.