Washington DC - President Donald Trump's deployment of thousands of National Guard troops in the US capital is unlawful, a federal judge ruled on Thursday.

District Court Judge Jia Cobb ordered an end to the deployment but stayed her order for 21 days to allow the Trump administration time to file an appeal.

The Republican president has sent National Guard troops to Democratic-run Washington, Los Angeles, and Memphis to combat crime and help enforce his aggressive crackdown on undocumented migrants.

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Federal judges have temporarily blocked the deployment of troops in two other Democratic-controlled cities – Chicago and Portland – and the Supreme Court is expected to deliver the final word soon on whether the actions are lawful.

Trump ordered more than 2,000 National Guard to patrol Washington on August 11, claiming the city was a "filthy and crime-ridden embarrassment."

Brian Schwalb, attorney general for the US capital, filed a lawsuit in September seeking to end the deployment of the National Guard in the city.

"Deploying the National Guard to engage in law enforcement is not only unnecessary and unwanted, but it is also dangerous and harmful to the District and its residents," Schwalb said.

Cobb, in her opinion, said the Trump administration had "acted contrary to law" by deploying the National Guard for "for non-military, crime-deterrence missions in the absence of a request from the city's civil authorities."

The judge, an appointee of Democratic President Joe Biden, also said the administration had overstepped its authority by bringing National Guard troops from out of state to patrol the capital.