A Tennessee state court judge has temporarily blocked Republican governor Bill Lee's deployment of National Guard troops to the city of Memphis, ruling that the use of troops was likely not legal under the state's militia law.

Patricia Head Moskal of the Davidson County Chancery Court in Nashville said the militia law required Tennessee's General Assembly to call in the National Guard for public safety, unless the state faced a rebellion or invasion.

Her order blocks the governor from "continuing the activation and deployment of Tennessee National Guard personnel" pending further court proceedings.

National Guard troops began patrolling Memphis as part of a federal task force to combat crime after Republican President Donald Trump requested the deployment in September.

State lawmakers and the mayor of Shelby County sued Lee to block the deployment, saying that the use of the troops for law enforcement violates the Tennessee constitution and state laws.

Memphis is one of several US cities where Trump has sent the National Guard departing from norms against deploying troops on US soil. Trump has said that they are needed to suppress civil unrest, support immigration enforcement and fight crime. Democrats have alleged the Republican president is abusing military powers meant for grave emergencies such as an invasion.

National Guard deployments to the Democratic-led cities of Los Angeles, Portland, Chicago and Washington, DC, also face legal challenges.

Three district court judges so far have ruled against Trump's use of troops in preliminary rulings, saying that the president cannot summon the military to respond to routine protests or engage in law enforcement. Two appeals courts have split on the issue, which is likely to ultimately be decided by the Supreme Court.

The situation in Memphis differs from the other cases, because the troops are being deployed by the Republican state governor and remain under the state's control. In other cities where Trump has deployed troops, Democratic governors and local officials have opposed the use of military forces, leading Trump to place those states' National Guard under the federal government's control.

The Memphis lawsuit is seeking to stop the troop deployment based on Tennessee state law, unlike the other cases which are based in federal law and the US Constitution. Memphis, a city of 611,000 along the Mississippi River, has one of the highest violent crime rates in the United States, FBI figures show. About 24 percent of residents live in poverty, more than double the national average, the US Census Bureau says. (Reuters)