Police said a former student opened fire at a Florida high school on Wednesday, killing at least 17 people and sending scores of students fleeing into the streets in the nation's deadliest school shooting since a gunman attacked an elementary school in Newtown, Conn.

Frantic parents rushed to Marjory Stoneman Douglas High School to find SWAT team members and ambulances surrounding the campus as classes prepared to dismiss for the day. Live footage showed emergency workers who appeared to be treating the wounded on sidewalks.

"It is a horrific situation," said Robert Runcie, superintendent of the school district in Parkland, Florida. "It is a horrible day for us."

The 19-year-old suspect was taken into custody without a fight about an hour after he left the scene, authorities said.

Broward County Sheriff Scott Israel said the suspect, identified as Nikolas Cruz, was previously expelled for disciplinary reasons. 

Police believe the shooter had at least one rifle and multiple magazines.

"It's catastrophic. There really are no words," Israel said on Twitter.

Most of the fatalities were inside the building, though some victims were found fatally shot outside, the sheriff said.

Dr Evan Boyar at Broward Health North told reporters Wednesday that eight victims and the suspect had been brought to his hospital. Boyar said two victims died, three were in critical condition and three were in stable condition. He said three patients were still in the operating room Wednesday evening. The suspect was treated and released to police.

Boyar said all the victims were shot but he declined to comment on their ages or the extent of their wounds.

He said eight other victims were taken to other hospitals, but he did not have information on their conditions.

'Get on the ground!'

The high school is a sprawling complex set on a tract in the South Florida community of Parkland, about 70 kilometres north of downtown Miami.

The school had just over 3,100 students in the 2016-17 academic year, according to the National Centre for Education Statistics. Major streets run along two sides and an expressway passes nearby on the other not far from a residential neighborhood of single family homes.

In a cul-de-sac near the school, Michael Nembhard was sitting in his garage when he saw a young man in a burgundy shirt walking down the street. In an instant, a police cruiser pulled up and officers jumped out with guns drawn.

"All I heard was 'Get on the ground! Get on the ground!'" Nembhard said. He said he could not see the young man's face, but that he got on the ground without incident.

The day started normally at the school, which had a morning fire drill, and students were in class around 2:30 pm ET when another alarm sounded.

Noah Parness, 17, said he and the other students calmly went outside to their fire-drill areas when he suddenly heard popping sounds.

"We saw a bunch of teachers running down the stairway, and then everybody shifted and broke into a sprint," Parness said. "I hopped a fence."

Beth Feingold said her daughter, Brittani, sent a text at 2:32 pm that said, "We're on code red. I'm fine," but sent another text shortly afterward saying, "Mom, I'm so scared."

Brittani later was able to escape the school, which is one of the largest in the state.

'I'm scared for the other parents here'

Inside the school, students heard loud bangs as the shooter fired. Many of them hid under desks or in closets and barricaded doors.

Television footage showed those students who fled leaving in a single-file line with their hands over their heads as officers urged them to leave the area quickly. Parents hurried to the scene.

Caesar Figueroa said when he got to the school to check on his 16-year-old daughter, he saw police officers drawing guns as they approached the campus.

"My wife called me that there was an active shooter, and the school was on lockdown. I got on the road and saw helicopters… It was crazy and my daughter wasn't answering her phone." She finally texted him that she was inside a closet with friends.

Len Murray's 17-year-old son, a student at the school, sent his parents a chilling text: "Mom and Dad, there have been shots fired on campus at school. There are police sirens outside. I'm in the auditorium and the doors are locked."

A few minutes later, he texted again: "I'm fine."

Murray said he raced to the school only to be stopped by authorities under a highway overpass within view of the school buildings. He said he told his son to save his battery and stop texting. The boy's mother told him to turn off his ringer.

Authorities told parents to gather at a nearby hotel to get information.

"I'm scared for the other parents here. You can see the concern in everybody's faces. Everybody is asking, `Have you heard from your child yet?"' Murray said.

Murray said he's had just one thought running through his mind since he got his son's text: "All I keep thinking about is when I dropped him off this morning. I usually say, 'I love you,' and I didn't this morning. He's 17, he's at that age, and I didn't say it this morning, and I'm just kicking myself right now over and over and over. Say it early and often, I'm telling you."

On Twitter, Florida Governor Rick Scott said he has spoken with US President Donald Trump about the shooting, and that both will receive briefings and updates from law enforcement.

Trump tweeted: "No child, teacher or anyone else should ever feel unsafe in an American school."