Several American schools were forced to cancel walkouts in protest against gun violence after receiving threats.

Walkouts were expected to begin at 10am in each time zone on Wednesday, and last 17 minutes ― one minute for each person who was killed in last month’s shooting at Marjory Stoneman Douglas High School, in Florida.

But a number of schools said they had abandoned their planned protests at the eleventh hour while police investigated threats.

Atascadero High School, in California, was told to “shelter in place” at 9.40am on Wednesday by Atascadero Police Department after officers received reports of a threat made on Instagram, according to The Washington Post

A student is said to have written: “Im [sic] shooting up the school tomorrow during the walkout. Are you? Im going to enjoy every scream and every tear. Are you?

“We are NOT going to stop. We are NOT going to give up. We will NOT stop till every living thing on the campus is dead,” the student wrote.

Officers attended the student’s home at 8.10am and confirmed he had made the social media post, but said he acted alone.

He was taken into police custody and then to San Luis Obispo Juvenile Services Center.

Elsewhere in California, Righetti High School is reported to have received a threat shortly before its planned walkout at 10am.

Bradley-Bourbonnais Community High School and Romeoville High School, both in Illinois, also announced that threats had led them to cancel planned walkouts the evening before.

In Wisconsin, Mukwonago High School and Pewaukee High School also cancelled their walkouts.

In a statement on Facebook, Mukwonago Area School District said police were investigating a rumour that someone was planning to open fire at the school during the walkout.

Nikolas Cruz is accused of murdering 17 people (AP)

After interviewing three students at their homes, police said they “determined that there was no validity to the rumours nor was there a credible threat to student safety” at the school.

Nikolas Cruz, 19, is alleged to have shot dead 14 pupils and three staff members at Parkland's Marjory Stoneman Douglas High School on Valentine’s Day, 14 February.

The teenager had been expelled from the school a year ago and was attending another local school.

Florida shooting – in pictures Show all 10 1 /10 Florida shooting – in pictures Florida shooting – in pictures Police arrest a suspect in connection with the shooting at the Marjory Stoneman Douglas High School in Parkland, Florida Reuters Florida shooting – in pictures Parents wait for news after reports of a shooting at Marjory Stoneman Douglas High School in Parkland, Florida AP Florida shooting – in pictures Anxious family members wait for news of students AP Photo/Wilfredo Lee Florida shooting – in pictures Students being evacuated from the school AP Florida shooting – in pictures Students being evacuated from the school Getty Florida shooting – in pictures People gather waiting for word from students AP Florida shooting – in pictures Parents waiting for news on their children AP Florida shooting – in pictures People gather at a hotel where students were taken after the shooting Getty Florida shooting – in pictures Florida Governor Rick Scott speaks to the media as he visits Marjory Stoneman Douglas High School following the shooting AFP/Getty Florida shooting – in pictures Dr. Igor Nichiporenko, Medical Director Trauma, left, and Dr. Evan Boyer, Medical Director, Emergency Services, speak about treating victims and the suspect at a press conference outside Broward Health North hospital AP

Officials have said the AR-15 semi-automatic weapon that Cruz is believed to have used in the attack was purchased legally.

There is said to have been a huge increase in the number of threats against US schools since the shooting last month.