Police are investigating "numerous bomb threats" made against four post-secondary institutions around Toronto on Tuesday.

In response, OCAD University was evacuated and closed all of its buildings this morning, while Ryerson University's Chang School was briefly closed but has since reopened.

The <a href="https://twitter.com/TorontoPolice?ref_src=twsrc%5Etfw">@TorontoPolice</a> Service is responding to numerous bomb threats to post-secondary campuses throughout the city. We're investigating & working w/ schools to evacuate where necessary. Anyone who has received a threat or has info on the investigation is asked to call police. ^CdK —@TPSOperations

Investigators say threats, "all similar in nature," were made to a total of four schools: OCAD U, Ryerson, Humber College and George Brown College.

So far, Humber and George Brown have opted not to evacuate, though George Brown did tweet that officials are "working with police" and that students should report any suspicious activity.

Update - 4 schools / 10 campuses involved.<br>- first call to <a href="https://twitter.com/TorontoPolice?ref_src=twsrc%5Etfw">@torontopolice</a> 8:54am<br>- threats all similar in nature<br>- these calls are taken very seriously<a href="https://twitter.com/OCAD?ref_src=twsrc%5Etfw">@OCAD</a> <a href="https://twitter.com/GBCollege?ref_src=twsrc%5Etfw">@GBCollege</a> <a href="https://twitter.com/RyersonU?ref_src=twsrc%5Etfw">@RyersonU</a> <a href="https://twitter.com/humbercollege?ref_src=twsrc%5Etfw">@HumberCollege</a> ^CdK —@TPSOperations

Within those four schools, 10 campuses were threatened, police wrote on Twitter.

Police said that the first call about the threats came in just before 9 a.m. on Tuesday, and that they take the calls "very seriously."

OCAD University announced just before 10 a.m. that all buildings were being evacuated and closed. In a follow-up statement on its website, the school wrote that it would remain closed through the entire day and evening for the police investigation.

Classes will resume on Wednesday at OCAD U following a police probe. (Jon Castell/CBC)

Police Chief Mark Saunders was asked about the threats at a press conference Tuesday morning.

He said that police take this kind of call seriously and are investigating.

OCAD U will reopen Wednesday and classes will resume their regular summer schedule, after police searched and cleared the downtown campus.