President Donald Trump could bring back a rarely used 19th century law to deploy active soldiers on city streets.

The US President left open the possibility that he might use the Insurrection Act to sidestep any court rulings blocking the dispatch of Guard troops into Democratic-led cities, over the objections of local and state officials.

A federal judge has temporarily barred Guard troops from heading to Portland, Oregon, though a separate judge has allowed a deployment to proceed in Chicago, where federal agents have embarked on a sweeping crackdown on illegal immigration.

"Well, it's been invoked before," Mr Trump told reporters in the Oval Office.

The Republican President has claimed troops are needed to protect federal property and personnel in carrying out their duties, as well as assisting an overall drive to suppress crime.

He said: "If you look at Chicago, Chicago is a great city where there's a lot of crime.

"If the governor can't do the job, we'll do the job.

"It's all very simple."

President Donald Trump has not ruled out bringing the law back

The law, which gives the president authority to deploy the military to quell unrest in an emergency, has typically been used only in extreme cases, and almost always at the invitation of state governors.

The act was last invoked by President George HW Bush during the Los Angeles riots of 1992.

Under federal law, National Guard and other military troops are generally prohibited from conducting civilian law enforcement.

However, the Insurrection Act allows for an exception, giving troops the power to directly police and arrest people.

Protesters gather in Los Angeles against immigration policy

Last week, in a speech to top military commanders, President Trump suggested using US cities as "training grounds" for the armed forces, alarming Democrats and civil liberties groups.

Randy Manner, a retired Army major general who served as acting vice chief of the National Guard Bureau, said using the Insurrection Act in the way Trump appears to be contemplating has no real precedent.

Mr Manner, who served under both Republican and Democratic administrations before retiring in 2012, said: "It's an extremely dangerous slope, because it essentially says the president can just do about whatever he chooses,"

"It's absolutely, absolutely the definition of dictatorship and fascism."

A Texas Army National Guard 36th Infantry Division soldier arrives in Chicago

President Trump has ordered Guard troops to Chicago, the third-largest US city, and Portland, Oregon, following his earlier deployments to Los Angeles and Washington DC.

In each case, he has defied staunch opposition from Democratic mayors and governors, who say Trump's claims of lawlessness and violence do not reflect reality.

Texas Guard troops were seen on Tuesday assembling at the Army Reserve Training Center in Elwood, about 50 miles southwest of Chicago.

It was not immediately clear when they plan to begin operations in the city itself.

In Chicago and Portland, protests over Mr Trump's immigration policies had been largely peaceful and limited in size, according to local officials, far from the "war zone" conditions described by President Trump.

Law enforcement officers deploy smoke grenades to disperse protesters gathered at the Ice Headquarters in Portland

Since the surge of federal agents to the Chicago area last month, the demonstrations have done little to upset life in a city where violent crime has fallen sharply.

Restaurants and theaters are as busy as ever, and crowds have flocked to lakefront beaches to enjoy an unusual stretch of warm weather.

Illinois Governor JB Pritzker, a Democrat, accused Mr Trump of intentionally trying to foment violence to justify further militarisation.

"Donald Trump is using our service members as political props and as pawns in his illegal effort to militarize our nation's cities," Governor Pritzker said.