The US state of Illinois has urged a judge to order the National Guard to stand down in the Chicago area, calling the deployment a constitutional crisis and suggesting the Trump administration gave no heed to the pending legal challenge when it sent troops to an immigration enforcement building.

The government "ploughed ahead anyway," lawyer Christopher Wells from the state attorney general's office said.

"Now, troops are here."

Wells' arguments opened an extraordinary hearing in federal court in Chicago on Thursday.

The city and the state, run by Democratic elected leaders, say President Donald Trump has exceeded his authority and ignored their pleas to keep the guard off the streets.

Heavy public turnout at the downtown courthouse caused officials to open an overflow room with a vid

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