The National Rifle Association has sued Florida after it passed a gun control law in the wake of a school shooting last month that left 17 people dead.
Governor Rick Scott, a Republican and staunch ally of the gun lobby, enacted the bill.
The law raises the legal age for buying rifles in Florida, but also allows the training and arming of school staff.
It does not ban semi-automatic rifles like the one used in the 14 February massacre in Parkland.
But it does introduce a three-day waiting period on all gun sales and a ban on bump stocks, a device that enables semi-automatic rifles to fire hundreds of rounds a minute.
The NRA filed its lawsuit on Friday just an hour after the bill was signed by the governor.
The complaint says the law violates the second amendment of the US constitution, which governs the right to bear arms.
It also argues the bill breaches the 14th amendment's equal protection clause by banning law-abiding citizens between 18-21 from buying guns.
The legal action says the Florida legislation particularly affects young women.
"Females between the ages of 18 and 21 pose a relatively slight risk of perpetrating a school shooting... or, for that matter, a violent crime of any kind," says the lawsuit.
The bill's passage by a Republican-controlled legislature in a state where the NRA wields considerable influence is seen as a testament to an impassioned pro-gun control campaign launched by young survivors of the shooting and parents of the victims.
Governor Scott signed the bill surrounded by Parkland students and families.
"Today should serve as an example to the entire country that government can and must move fast," he said.
A former student with a history of mental health issues is accused of killing 14 students and three staff members at Marjory Stoneman Douglas High School on 14 February in the second-deadliest US school shooting ever.
The Florida bill is also controversial because it allows school staff to be specially trained to carry guns as campus "guardians".
However, local sheriffs and individual school districts may opt out and most classroom teachers will be excluded from the scheme.
Six other states - Wyoming, South Dakota, Tennessee, Georgia, Kansas and Texas - allow public school employees to carry firearms to work, according to the National Conference of State Legislatures.
President Donald Trump has voiced support for arming teachers and so has the NRA.
The Florida bill also included a provision to make it easier for police to confiscate weapons from anyone found to pose a threat of violent behaviour.
This so-called "red flag" measure has also been proposed by five other states - Rhode Island, Utah, Kentucky, Alabama and Pennsylvania - since last month's Parkland attack, according to the Brady Campaign to Prevent Gun Violence.