NEWTOWN, Connecticut (Reuters) - A heavily armed gunman opened fire inside a Connecticut elementary school on Friday, killing 26 people, including 20 children, police said, in one of the worst mass shootings in U.S. history.

The gunman - who according to a media report carried four weapons and wore a bullet-proof vest - was dead inside Sandy Hook Elementary School in Newtown, Connecticut, state police Lieutenant Paul Vance told a news conference.

Vance said authorities found 18 children and seven adults, including the gunman, dead at the school, and two children were pronounced dead later at a hospital. Another adult was found dead at a related crime scene in Newtown, he said, 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 his mother and then 18 students in the room before shooting five other adults and killing himself.

"Our hearts are broken today," President Barack Obama said in an emotional televised address to the nation.

Chaos struck as children gathered in their classrooms for morning meetings at Sandy Hook Elementary in Newtown, a city of about 27,000 in northern Fairfield County, about 45 miles southwest of Hartford and 80 miles northeast of New York City.

The holiday season tragedy was the second shooting rampage in the United States this week and the latest in a series of mass killings this year, and was certain to revive a debate about U.S. gun laws.

Obama, wiping away tears and pausing to collect his emotions, mourned the "beautiful little kids between the ages of 5 and 10 years old" who were killed.

"As a country we have been through this too many times," Obama said, ticking off a list of recent shootings.

"We're going to have to come together and take meaningful action to prevent more tragedies like this, regardless of the politics," Obama said in apparent reference to the influence of the National Rifle Association over members of Congress.

Obama remains committed to trying to renew a ban on assault weapons, White House spokesman Jay Carney said.

BLOODIED CHILDREN LEAVE SCHOOL

Vance said the shootings took place in two rooms of Sandy Hook Elementary School, which teaches children from kindergarten through fourth grade, roughly ages 5 to 10.

Witnesses reported hearing dozens of shots; some said as many as 100 rounds.

"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.

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

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

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

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

The toll exceed that of one of the most notorious U.S. school shootings, the 1999 rampage at Columbine High School in Littleton, Colorado, where two teenagers killed 13 students and staff before killing themselves.

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 on camera.

In Hoboken, New Jersey, police cordoned off a block in connection with the Connecticut shootings but an officer told reporters there was no body inside, contrary to an earlier media report.

The United States has experienced a number of mass shooting rampages this year, most recently in Oregon, where a gunman opened fire at a shopping mall on Tuesday, killing two people and then himself.

The deadliest came in July at a midnight screening of a Batman film in Colorado that killed 12 people and wounded 58.

In 2007, 32 people were killed at Virginia Tech university in the deadliest act of gun violence in U.S. history.

In another notorious school shooting outside of the United States, in 1996 a gunman opened fire in an elementary school in Dunblane, Scotland, and killed 16 children and an adult before killing himself.