President Donald Trump on Monday reiterated his position that people entering the U.S. from Mexico — individuals he had compared to invaders — should be immediately sent back without appearing before a judge.

The American Civil Liberties Union says that such a step would be illegal and violate the Constitution that Trump swore to uphold.

Trump said on Twitter that he opposed hiring "many thousands" of judges to deal with immigration cases and said people should just be refused entry at the border.

Hiring manythousands of judges, and going through a long and complicated legal process, is not the way to go - will always be disfunctional. People must simply be stopped at the Border and told they cannot come into the U.S. illegally. Children brought back to their country...... —@realDonaldTrump ....If this is done, illegal immigration will be stopped in it’s tracks - and at very little, by comparison, cost. This is the only real answer - and we must continue to BUILD THE WALL! —@realDonaldTrump

Trump had said on Sunday that border crossers should not be heard in court.

"We cannot allow all of these people to invade our Country," the president said on Twitter as he was being driven to his private golf club in Northern Virginia. "When somebody comes in, we must immediately, with no Judges or Court Cases, bring them back from where they came. Our system is a mockery to good immigration policy and Law and Order."

"Most children come without parents ... Our Immigration policy, laughed at all over the world, is very unfair to all of those people who have gone through the system legally and are waiting on line for years!" he continued. "Immigration must be based on merit - we need people who will help to Make America Great Again!"

'Illegal and unconstitutional'

"What President Trump has suggested here is both illegal and unconstitutional," said Omar Jadwat, director of the ACLU's Immigrants' Rights Project. "Any official who has sworn an oath to uphold the Constitution and laws should disavow it unequivocally."

Trump has been criticizing immigration judges for weeks, both exaggerating the number currently hearing cases and saying that hiring more — as some members of Congress have proposed — would be unnecessary.

A Trump supporter and a protester scream at each other during a march to a shelter for unaccompanied children in Florida on Saturday. (Brynn Anderson/Associated Press)

Trump made his anti-illegal immigration stance a centrepiece of his presidential campaign and he has pushed for strict policies since taking office. He said during a campaign appearance Saturday in Las Vegas that being for "strong borders, no crime" is a winning issue for Republicans to run on in November's congressional elections.

But he bowed to public pressure last week and reversed a policy of separating adults and children who enter the U.S. illegally together at the border with Mexico, though his "zero-tolerance" policy of criminally prosecuting all illegal border-crossers remains.

'Trump Should Be Caged'

The House is expected to vote on immigration legislation this week, though its fate is uncertain.

Trump told House Republicans last week that he was "100 per cent" behind the effort but tweeted days later that they were "wasting their time" voting before the midterm elections.

About a dozen protesters gathered at the entrance to Trump's club Sunday afternoon as he prepared to leave, including a woman holding a sign that said "Trump Should Be Caged" and a man wrapped in a Mylar blanket. Some of the separated children were seen using the blankets on government-distributed video of their holding conditions.

Further up the road, a lone man stood with a sign with Trump's headshot and the words "Thank You."