Police are investigating what they call a "threat" that forced the evacuation of more than 1,000 students, staff and faculty Friday from a health sciences building at McMaster University.

The Institute for Applied Health Sciences (IAHS) building on campus was closed around 10:30 a.m. because of a threat against the building. Police have not indicated the nature of the threat.

All classes and activities scheduled in the building have been cancelled. The university announced that faculty, staff and students have been sent home.

Katie Clarke was waiting for the Remembrance Day assembly to start in the main lobby when she was told she had to leave.

"One of the instructors just stopped and said 'you all must leave this area,'" Clarke said.

Clarke, a registered practical nursing student, waited around for over an hour to be let back in to retrieve her belongings.

Police say they received a call to investigate the threat just before 9 a.m. Friday morning.

"The building has been closed down," said Wade Hemsworth, director of public relations for the university.

"As a result the faculty, staff and students have been cleared out and sent home for the day," he said. "Classes and activities there are cancelled for the day and updates about the building will be posted on our daily news site and social media accounts as soon as they happen."

Police were still searching the building around noon.

The IAHS building is a joint building between McMaster and Mohawk College. Mohawk spokesperson Jay Robb estimated there would have been about 1,200 students inside when the threat was made.