US President Donald Trump threatened on Saturday to unleash his newly rebranded "Department of War" on Chicago, further heightening tensions over his push to deploy troops into Democratic-led US cities.

The move seeks to replicate an operation in Washington DC, where Trump deployed National Guard troops and boosted numbers of federal agents, sparking a backlash and a fresh protest on Saturday that drew thousands.

"Chicago about to find out why it's called the Department of WAR," Trump posted Saturday on his Truth Social account.

The Democratic governor of Illinois, where Chicago is located, voiced outrage at Trump's post.

"The President of the United States is threatening to go to war with an American city. This is not a joke. This is not normal," Governor JB Pritzker wrote in a post on X.

"Illinois won't be intimidated by a wannabe dictator," he added.

The 79-year-old Republican has steadily ramped up threats against Chicago, since an early mention of it at the end of August.

Anti-Trump protesters took to the streets of Chicago on Saturday, carrying signs that read "stop this fascist regime!" and "no Trump, no troops."

The protest route also went past Chicago's Trump Tower, and protesters made rude gestures at the president's building as they walked past.

In addition to Chicago, Trump has threatened to replicate the surges in Democratic-led Baltimore and New Orleans.

On Friday, Trump signed an order changing the name of the Department of Defence to the Department of War, saying it sends "a message of victory" to the world.

Pentagon chief Pete Hegseth cheered the move, saying the US will decisively exact violence to reach its aims, without apology. (AFP)