Donald Trump is set to become the first former or sitting US president to face criminal charges. He will enter a plea in a criminal case at a New York City courtroom after a grand jury indicted him last week.

He spent the night in Trump Tower in central Manhattan.

The charge, or charges, against him will not be made public until his arraignment Tuesday afternoon.

But they're expected to involve allegations Trump paid hush money to an adult film actress during his 2016 election campaign.

New York police have tightened security in the streets surrounding the courthouse. They've warned against any disturbances from Trump's supporters or his opponents.

Meanwhile, some of those supporters gathered outside Trump Tower. Many agree with his claims the case is a political "witch hunt."

One of the backers said, " I don't think he's above the law. But there's a lot of people, a lot of lawless people in New York City right now that should be arrested. And anybody who's a victim of a crime in New York City that didn't get the justice they deserve should be outraged today at what the D.A. is using all his resources on."

Another Trump partisan said, "I will forever support this beautiful man who saved our country one time. And yes, he's going to do it again. God bless president Trump. Best president this country ever had."

Questions have been raised over whether the case will impact Trump's prospects for next year's election. A recent poll conducted by Yahoo News and YouGov found his approval rating among Republicans rose after the indictment.

Speaking to NHK, an expert says Trump will likely try to prolong the trial and spin it for political gain.

Professor of Keio University, Watanabe Yasushi, says, " Trump may question the structure of the grand jury, the district attorney's political stance and his past words and actions to claim that this is an unjust legal procedure being conducted by people appointed unjustly. He will likely claim the indictment is invalid."

After entering his plea, Trump is expected to be granted bail. He's then scheduled to return to his home in Florida where he'll deliver a speech.