Students place flowers on the stage outside the Pines Trail Center where a candlelight vigil will be held in Parkland, Florida. Picture: Joel Auerbach/AP

Florida - The anguished mother of a girl slain with 16 other people by a gunman at a Florida high school took to national television on Thursday to angrily demand that US President Donald Trump "do something" to protect school children from gun violence.

"President Trump, you say what can you do? ... You can do a lot," Lori Alhadeff shouted into the camera in an emotionally raw appearance on CNN. Fighting tears, her voice quaking with anger, she screamed, "Do something. Action. We need it now. These kids need safety now."

The mother said she had just spent two hours making funeral arrangements for her 14-year-old daughter Alyssa Alhadeff, who was one of 17 people shot dead on Thursday at Marjory Stoneman Douglas High School in Parkland, Florida, a suburb of Miami.

"How do we allow a gunman to come into our children’s school? How did they get through security? What security is there?" she cried.

"The gunman, the crazy person, just walks right into the school, knocks down the window to my child’s door and starts shooting, shooting her ..."

A 19-year-old former student of the school, Nikolas Cruz, has been accused of the murders.

The mother's impassioned remarks left CNN's anchor, Brooke Baldwin, who was broadcasting outside the school the day after the shooting, momentarily speechless as she choked back tears.

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Earlier on Thursday Trump addressed the shooting in a White House speech that emphasised school safety and mental health but made no mention of gun policy.

"President trump, you say what can you do? You can stop the guns from getting into these children's hands. Put metal detectors at every entrance to the schools," Alhadeff screamed. "President Trump, please do something."

Alhadeff's comments spoke to growing frustration in the United States on the part of parents because of the lack of action by policymakers in Washington regarding gun control, especially regarding assault rifles. An AR-15 style rifle was used in Wednesday's shooting.

Including Thursday's massacre at Marjory Stoneman Douglas High School, there have been 18 school shootings since January. 1 this year.