Highgate School physically chained its gates shut after a online messageboard user warned he was about to “kill as many children as I can” in a hoax terror threat.

Police swarmed into the North Road school at 12.30pm in response to the message posted online, which warned that hundreds of pupils were about to be killed.

The school went into lockdown and kept pupils inside until police declared that the incident was a hoax following an investigation.

The post read: “Today is the day. In 25 minutes, the bells will ring at Highgate school, I will kill as many children as I can for the countless children you have killed.”

The post was signed off “Allahu Akbar my brothers and sisters”.

The message was accompanied by a photograph of several guns on top of a bed.

The school is no longer in lockdown but police will be keeping a “high-profile” presence at the school for the rest of the day.

A spokeswoman for Highgate School said: “Earlier today, Highgate School received information from an Internet message board indicating a possible threat to the school.

“Highgate immediately contacted the police who arrived on site, while the school’s emergency lockdown procedures were activated to ensure the safety of pupils and staff.

“Following reassurance from the police who had investigated the credibility of the threat, these lockdown measures have now have been lifted.

“While police will be maintaining a high profile around the school for the remainder of the day, they have advised that normal routines can now resume.”

Eyewitness Casey Atchley was due to attend a meeting at the school, but when he tried to enter the building, he was told by security: “You are not coming in right now”.

The 27-year-old outreach coordinator at an educational travel company said: “I heard a lot of sirens and then I saw about five or six police cars come in.

“I went on Twitter and I saw a single Tweet about something being posted online.”

He added: “It was tense. I didn’t know it was a hoax.”