UPDATED 7.38PM: Invercargill residents are being asked to remain vigilant as a gunman remains on the loose.

A man armed with a shot gun and wearing a Halloween mask robbed a Four Square in the outlying suburb of Otatara this afternoon.

He took off on foot.

Police temporarily locked down a local school and asked residents to lock their homes and sheds.

They brought in the Armed Offenders Squad and a helicopter, but the gunman remains on the run.

Police say it's possible he is no longer in the area.

However they are asking residents to immediately report any suspicious activity, and to use common sense when it comes to their safety.

They have posted CCTV footage of the robbery on the Southland Police Facebook page.