Attendees visit the trade booths during the National Rifle Association's annual convention in Louisville, Kentucky.

You can't take knives to the National Rifle Association's gun expo in Kentucky, US.

Many attending the expo found out the hard way, and were surprised to find they couldn't bring blades in the event, NY Daily News reported.

Thousands are attending the convention, which got under way on Friday, where Republican presidential nomination Donald Trump spoke.

JOHN SOMMERS/REUTERS NRA members stand in long lines as the shop at the NRA Store at the National Rifle Association's annual exhibition in Louisville, Kentucky.

Chairman of the advocacy groups Knife Rights said he was happy to help the the NRA, so set up a check-in tent for those with knifes.

"For whatever reason the Secret Service doesn't want knives in and we're happy to help," Ritter said.

By Friday afternoon about 3000 knives had been left at the check-in.

AARON P. BERNSTEIN/REUTERS Attendees look on as the national anthem is sung before the National Rifle Association's NRA-ILA Leadership Forum during their annual meeting in Louisville, Kentucky.

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Veteran NRA member Don Huskey praised the knife check-in.

"People forget knives and almost every good ol' boy and girl still have knives. I forgot I had it on me. It happens to be an NRA knife. So it's kind of sentimental. So I didn't want to let it go."

The gun expo - dubbed "Freedom's Safest Place" by the NRA - has more than 800 exhibits, featuring an array of firearms and firearms related products, according to local new site WKLY.com.

"Over the next three days we are going to see more than 70,000 people come down to the Kentucky Expo Center," NRA spokesman Lars Dalseide said.