Gun and ammo stores near Lewiston, Maine are seeing a huge surge in folks trying to buy firearms and bullets, with the majority of people coming in to buy handguns.

First Due Firearms tells TMZ ... the phone is ringing off the hook with lots of first-time gun buyers and others looking to add to their firearm collection, with many folks ready to purchase and put money down.

The store says buyers are citing Wednesday's mass shooting, which killed 18 people, as the reason for purchasing guns and ammo.

Over at G3 Firearms, we're told the store's been packed since it opened bright and early at 9 AM ... with people wanting guns for protection. Most of the sales were for handguns, but we're told a few folks bought AR-15s and some even asked for body armor.

Meanwhile, at 3 Cousins Firearms we're told the business is being swamped with calls, emails and Facebook messages from people wanting guns ... and they expect to be super busy the rest of the week.

At another gun store in the area, we're told there was a 50% increase in firearms and while interest was largely for handguns, we're told a couple rifles were sold too.

With multiple counties in Maine in lockdown amid the manhunt for alleged gunman Robert Card, it sounds like people are arming themselves and bracing for the worst.