A former student opened fire at a Florida high school on Wednesday, killing 17 people and sending students running out into the streets. SWAT team members swarmed in before authorities took the gunman into custody.

Frantic parents rushed to the scene and ambulances converged in front of Marjory Stoneman Douglas High School. Live footage showed emergency workers appearing to treat wounded people on the sidewalks.

The Broward County Sheriff's Office tweeted that 17 people had been shot dead by the gunman.

Later on, they added: "Victims have been and continue to be transported to Broward Health Medical Center and Broward Health North hospital."

"It's catastrophic. There really are no words," said Sheriff Scott Israel.

Broward County Schools Superintendent Robert Runcie said there were "numerous fatalities."

"It is a horrific situation," he added. "It is a horrible day for us."

US President Donald Trump tweeted: "My prayers and condolences to the families of the victims of the terrible Florida shooting. No child, teacher or anyone else should ever feel unsafe in an American school," he wrote.

The male shooter was identified as Nikolas Cruz, 19, according to a US official briefed on the investigation. Cruz was a a former student who had been expelled for "disciplinary reasons." He used a rifle but may have also had a handgun.

"He always had guns on him and stuff like that," said a student who knew Cruz. "He was a little bit of a troubled kid."

Cruz was taken into custody "without incident" about an hour after he left the school campus, officials said.

Sheriff Israel said police were waiting for the SWAT team to give them the all-clear so that they could go inside.

Noah Parness, a 17-year-old junior, said the fire alarm went off for the second time of the day at about 2.30pm (2.30am Thursday, Thailand time). He and the other students calmly went outside to their fire drill areas when he suddenly heard several pops.

"Everyone was kind of just standing there calm, and then 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."

Television footage showed police with weapons drawn entering the school, then dozens of students running and walking quickly out.

A police officer waved the students on, urging them to quickly evacuate. Some students exited the building in single-file rows with hands raised overhead to show they carried no weapons. Others held onto other students as they made their way out past helmeted police in camouflage with weapons drawn.

The Broward Schools department said on its website that students and staff heard what sounded like gunfire and the school immediately went into lockdown.