Washington (CNN) President Donald Trump announced on Friday that his administration will issue a new rule banning bump fire stocks, a gun accessory that makes it easier to fire rounds quickly from a semi-automatic weapon, mimicking automatic fire.

The announcement comes a day before large numbers of protesters are expected to descend on Washington, DC for "March for Our Lives," a rally organized to push for stricter gun laws in the wake of the school shooting in Parkland, Florida last month.

"Obama Administration legalized bump stocks. BAD IDEA. As I promised, today the Department of Justice will issue the rule banning BUMP STOCKS with a mandated comment period," Trump wrote. "We will BAN all devices that turn legal weapons into illegal machine guns."

Shortly after Trump's tweet, Attorney General Jeff Sessions announced that the Justice Department would clarify rules that define bump stocks within the definition of "machine gun" under federal law, a move that would ban the sale of such accessories.

Trump directed his attorney general to propose changes that would ban bump stocks in February. The move came four months after a gunman in Las Vegas used a bump stock to fire down on concertgoers, killing 58 people, the deadliest mass shooting in modern American history.

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