A gunman opened fire at a Florida high school on Wednesday, causing "multiple" fatalities, according to officials, and leaving terrified students huddled in their classrooms, texting friends and family for help.

Broward County Sheriff Scott Israel said the Valentine's Day shooting at Marjory Stoneman Douglas High School in Parkland, had left "multiple dead."

Fourteen people were taken to hospital "with varying degrees of wounds," Israel told reporters, adding that the school had not yet been totally cleared.

The suspect -- a onetime student at the school who was about 18 years old -- was arrested without incident in the nearby town of Coral Springs, he said.

The shooting, one of nearly 20 since the start of the year, will once again throw the spotlight on the epidemic of gun violence in the United States and the ready accessibility of weapons in a country with 33,000 gun-related deaths annually.

"Just a horrible day for us," said the superintendent of the county's school district, Robert Runcie, who spoke of "numerous fatalities."

Israel echoed that sentiment.

"This is a terrible day for Parkland," he said, speaking of the city of about 30,000 people, located 50 miles (80 kilometers) north of Miami.

"My very own triplets went to that school."

The FBI said it was assisting local law enforcement with the investigation.

'Very scared'

Parkland Mayor Christine Hunschofsky told CNN she had spoken to a number of students after the shooting erupted shortly after 2:00 pm (1900 GMT).

"They were very scared," she said. "And almost in shock when they came out."

Asked about security, the Parkland mayor said a police officer is always stationed at the school and there was a "single point of entry."

Television images showed students being led out of the school by heavily armed police officers and an armored vehicle filled with a SWAT team on the scene.

One injured victim was seen being placed into an ambulance on a stretcher.

Police officers in helmets, bulletproof vests and armed with automatic weapons could be seen stationed at several points around the sprawling school complex, which serves nearly 3,000 students.

'Everyone started running'

Student Jeiella Dodoo told CBS News that she and her schoolmates had evacuated their classroom calmly after hearing what they thought had been a routine fire alarm.

"The alarm went off so we had to evacuate from our classes," she said. "Then we heard gunshots.

"I heard about six gunshots," she said, "and then some people started running and then everyone started running because we were like 'If it's real, then just run.'"

A math teacher at the school told CBS that he was hiding with six of his students.

"We are fine. We are waiting," he said.

Caesar Figueroa told CNN his daughter was hiding in a closet and texting her family.

"She's trapped with her 10 friends. She said she heard gunshots. A window blew and everybody is screaming and running, and she said she ran in the closet and she's still there," Figueroa said.

No child should 'feel unsafe'

"My prayers and condolences to the families of the victims of the terrible Florida shooting," President Donald Trump said on Twitter.

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

Since January 2013, "there have been at least 283 school shootings across the country -- which averages out to one school shooting a week," according to Everytown for Gun Safety, a non-profit group that advocates for gun control.

Since the 2012 massacre at the Sandy Hook Elementary School in Newtown, Connecticut, where 20 children and six adults were shot dead, warning procedures and emergency drills have multiplied at US schools.

The goal is to teach school children how to react to a shooter who opens fire at random.