Between 20-50 people were injured and “multiple” people were killed as students frantically hid while a gunman stormed the building.
Footage has emerged appearing to be from inside the school with gunshots heard in the background.
Students could be heard screaming and were seen kneeling on the floor and hiding under desks as the gunman opened fire at Marjory Stoneman Douglas High School in Parkland.
Margate Fire Department told CBS Miami between 20 and 50 people have been injured while other sources say “at least seven” people were killed, although these figures remain unconfirmed.
The suspected gunman, believed to be wearing a gasmask, opened fire at Marjory Stoneman Douglas High School shortly after 2pm local time (7pm GMT) just as students were preparing to leave for the day.
The suspect is now in police custody, according to a spokesman for the Broward County Sheriff.
Police believe the suspect is a student at the school and said he was wearing a hoodie and has red hair and freckles.
He is also believed to be a member of the US Army Junior Reserve Officers’ Training Corps program.
A SWAT team surrounded the school in Parkland to try to control the situation after the school had been placed on “code red” lockdown.
School Superintendent Robert Runcie said: “I can’t confirm the number at this point but there are numerous fatalities here.
“It’s a horrific situation. It’s just a horrible day for us. We have no evidence that there has been more than one shooter. I believe there is an administrator or a teacher who is involved, there may be more.
“It’s a day that you pray every day that you will never have to see. It is in front of us and I ask the community for their prayers, their support for these children and their families.
“We’re doing everything we possibly can. The students are being evacuated and released at the moment so we’re working through that process right now.”
The White House confirmed President Donald Trump has been made aware of the incident and he has since 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.”