Donald Trump has been branded a "piece of s***" by a girl believed to be a student at the Florida school where 17 people were killed in a mass shooting.

The US President was tackled for his response on Twitter to the tragedy by 'Sarah', who urged him to "do something instead of sending prayers".

The girl, whose profile says she is 16, tweeted on Wednesday: "THERE'S A SCHOOL SHOOTING RN [right now] AT MY SCHOOL" as news broke of a gunman entering Marjory Stoneman Douglas High School in Parkland, Florida.

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THERES A SCHOOL SHOOTING RN AT MY SCHOOL — sarah (@chaddiedabaddie) February 14, 2018

She said she was "so f****** scared" and later claimed she had been evacuated.

"We're (sic) heard gun shots and the paramedics were everywhere, apparently multiple people got shot and I still have friends that are stuck inside the school," one post read.

The gunman was eventually captured and 19-year-old student Nikolas Cruz has been arrested as the main suspect.

Image: The suspect posed for guns on social media Pic: cruz.nikolaus/Instagram

Mr Trump has only commented on Twitter on the shooting, which started at around 3pm local time (8pm GMT).

He wrote: "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."

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. — Donald J. Trump (@realDonaldTrump) February 14, 2018

But Sarah sent an emotional reply back, calling for the US' latest mass killing to be the catalyst for action on gun control.

She wrote: "I don't want your condolences you f****** price [sic] of s***, my friends and teachers were shot. Multiple of my fellow classmates are dead.

"Do something instead of sending prayers. Prayers won't fix this. But Gun control will prevent it from happening again."

Image: The tweet 'Sarah' sent in response to Donald Trump's message

Her message garnered over 200,000 "favourites" on Twitter and sparked a fierce debate from other users about the state of America's gun control laws.

Some of the reaction from both sides included:

Details are still emerging about the shooting, but it is believed the gunman deliberately set off the fire alarm so youngsters would leave their classrooms as he arrived at the scene.

Twelve people were shot dead in the school building, two outside, one in the street and two people were left with injuries from which they later died.

Among those who tried to save students was football coach Aaron Feis, who died after apparently using his body as a shield against the bullets.