The injunction filed in the Cole County Circuit Court seeks to overturn the recently-signed "Second Amendment Preservation Act" that prevents local authorities from enforcing federal gun control laws.
Under the new law signed by Republican Governor Mike Parson, state and local law enforcement agencies can be fined about $50,000 per any officer who knowingly enforces federal gun law. It also "voids" any federal law, executive order, or regulation to track or remove firearms from citizens in Missouri.
Democrat mayor of St Louis, Tishaura Jones, said 2020 was the deadliest year for gun violence in the state’s history, and the deadliest in St Louis in half a century.
"And now the Missouri legislature is throwing up barriers to stop police from doing their most important job —preventing and solving violent crime,” she said in a statement to KMOV4.
“This harmful and unconstitutional law takes away tools our communities need to prevent gun violence."
Philip Dupuis, police chief in the St Louis suburb of O’Fallon, resigned in protest over the new law, saying the poorly worded language removes sovereign immunity and allowed officers to be sued for good faith seizures of firearms in emergency circumstances.
Mr Parson, however, said the law was designed to protect "law-abiding Missourians" against government overreach and unconstitutional federal mandates.
"We will reject any attempt by the federal government to circumvent the fundamental right Missourians have to keep and bear arms to protect themselves and their property," he said in a statement.
The injunction, filed against Missouri and the state’s Attorney General, Eric Schmitt, that the law, HB 85, violates the US Constitution’s Supremacy Clause, which provides that federal law preempts state law.
It says the law also violates the Missouri Constitution in several ways, including an infringement of the separation of powers of the branches of state government.
"In misguided seal to prevent imaginary threats to the right to keep and bear arms, the political branches in our state government blatantly violated the federal and state constitutions by attempting to nullify federal gun laws," the lawsuit says.
"The consequences of HB 85 are tangible and real: they will make it easier for criminals to use guns in committing violent acts, they will give gun violence a safe haven in Missouri, local governments… may be disqualified from receiving federal grants and technical assistance through the United States Department of Justice."
The Department of Justice, for its part, warned Missouri officials that the US Constitution’s Supremacy Claus trumped the new bill signed into law on Saturday.
Acting assistant attorney general Brian Boynton wrote in a letter to the governor that the law would disrupt the working relationship between federal and local law enforcement agencies.
The Associated Press contributed to this report.