Dozens of police officers, including a tactical response team, surrounded Alex Ferguson School on Friday after receiving a 911 call from a witness who said he saw someone carrying what appeared to be a gun.

The elementary school was surrounded just after 4 p.m. MT while police blocked off 26th Street S.W. at 17th Avenue.

A police helicopter circled above the southwest community of Shaganappi but officers were not able to locate the person.

"It's just so scary, you're so helpless. It's a scary, sickening feeling," said Layne Choong, the mother of a child who attends the school.

Calgary police say the incident started when a woman walking with three children near the school on Friday called 911 to report seeing a male, described as being about 17, wearing all black clothing including a balaclava and fingerless gloves.

The witness has described the gun as being an assault rifle with a red tip.

Police are asking for help from the public to identify the man.