AT LEAST 17 are were shot dead after a gunman openend fire at a high school in south east Florida.

The Broward County Sheriff's Office confirmed 19-year-old former student Nikolaus Cruz who had been expelled was arrested an hour and a half after the authorites were alerted to the shooting at the Marjory Stoneman Douglas High School in Parkland.

Broward County Sheriff Scott Israel said Cruz had used a rifle and had "countless magazines".

He said the shooting spree began outside the school where three people were killed. Cruz then entered the building and killed 12 people.

Two people later died after being taken to hospital.

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Aerial footage showed a number of people lying on the ground outside the school, being treated for injuries and moved to ambulances.

Cruz was taken into custody off campus about an hour after he "committed this horrific, detestable act," Israel said. 

"This is a terrible day for Broward County, the state of Florida, the United States," he said. "There really are no words."

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One student witness Michael Katz who was hiding in a classroom said: "I heard the shots. There were too many to tell. There was three people dead, two outside my door of my classroom. Literally outside my door.  I just heard screaming.

"The door was locked and all the students were low to the ground.  We were all safe."

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Students were seen filing away from the scene with their hands in the air as heavily armed SWAT teams members swarmed around the school.

"There are reports of victims," the sheriff's office added urging people to avoid the area.

Broward County Public Schools said on Twitter that the sound of gunfire broke out shortly before school was dismissed for the day, and that it received reports of "possible multiple injuries."

Another witness Zackary Wells said: "From what I heard he dropped his rifle and made his way out with other students that were being evacuated and arrested.

"I was leading some kids out from the classroom and I heard two or three shots.. we ran over to the classroom and barricaded ourselves in there. I am now on the street trying to get home safe."

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Coral Springs Police department, meanwhile, tweeted instructions to teachers and students to "remain barricaded inside until police reach you".

Footage from the scene showed students streaming away from the school. Some ran, while others walked quickly in orderly lines with their hands on or above their heads.

Dozens of law enforcement vehicles are on site, including one tank, along with a team of heavily armed law enforcement officers.

The high school went on immediate lockdown, Parkland Mayor Christine Hunschofsky said. 

"It’s sad. It’s sad that these tragedies happen in our country," she said. "Many of the students have been in touch with their parents. We have, many many parents out here."

President Trump has been briefed on the incident and the White House was monitoring the situation, White House press secretary Sarah Huckabee Sanders said.

HeraldScotland:

A parent of a student told the station that her son was hiding inside a closet at the school with classmates, and that teachers had told them not to leave the building.

Joel Leffler, who has a son and a daughter at the school, said both of his children were safe but in shock.

"My daughter, who was there in the freshman hall where the shooting took place — she's in shock right now and she's being taken out by SWAT.  She saw multiple dead bodies."