His indictment by a Manhattan grand jury is expected, but its timing is unclear.

With former President Donald J. Trump facing indictment by a Manhattan grand jury but the timing of the charges uncertain, he declared on his social media site that he would be arrested on Tuesday and demanded that his supporters protest on his behalf.

Mr. Trump made the declaration on his site, Truth Social, at 7:26. a.m. on Saturday in a post that ended with, “THE FAR & AWAY LEADING REPUBLICAN CANDIDATE AND FORMER PRESIDENT OF THE UNITED STATES OF AMERICA, WILL BE ARRESTED ON TUESDAY OF NEXT WEEK. PROTEST, TAKE OUR NATION BACK!”

Two hours later, a spokesman issued a statement clarifying that Mr. Trump had not written his post with direct knowledge of the timing of any arrest.

“President Trump is rightfully highlighting his innocence and the weaponization of our injustice system,” the statement said.

A lawyer for Mr. Trump, Susan R. Necheles, said that his post had been based on news reports, and accused the Manhattan district attorney’s office of conducting a “political prosecution.”

A spokeswoman for the Manhattan district attorney’s office declined to comment.

Prosecutors working for the district attorney, Alvin L. Bragg, have signaled that an indictment of Mr. Trump could be imminent. But they have not told Mr. Trump’s lawyers when the charges — expected to stem from a 2016 hush money payment to a porn star — would be sought or when an arrest would be made, people with knowledge of the matter said. At least one more witness is expected to testify in front of the grand jury, which could delay an indictment, the people said.

One of the people said that even if the grand jury were to vote to indict the former president on Monday, a Tuesday surrender was unlikely given the need to arrange timing, travel and other logistics.

The statement from Mr. Trump’s spokesman did not explain how he landed on Tuesday as an arrest date. One person with knowledge of the matter said that Mr. Trump’s advisers had guessed that it could happen around then, and that someone might have relayed that to the former president.

Mr. Trump, who declared his third presidential campaign in November and is leading his Republican opponents in most polls, faced his first criminal investigation in the late 1970s. He has been deeply anxious about the prospect of arrest, which is expected to include being fingerprinted, one of the people said.

When the Trump Organization’s former chief financial officer, Allen H. Weisselberg, was arrested in 2021, Mr. Trump watched in horror as television news showed Mr. Weisselberg flanked by officers in the courthouse and the former president said he couldn’t believe what was happening.

Mr. Trump’s post urging his supporters to protest and reclaim the nation carried unmistakable echoes of the incendiary messages he posted online in the weeks before the attack on the U.S. Capitol. In the most notorious of those messages, he announced on Twitter that he would hold a rally in Washington on Jan. 6, 2021. “Be there,” he told his millions of followers. “Will be wild.”

At that rally, on the Ellipse near the White House, Mr. Trump told supporters to march to the Capitol, where the certification of the 2020 presidential election was taking place. He is under investigation by federal prosecutors for his activities before the attack.

Investigators later determined that far-right extremist groups as well as ordinary Trump supporters had read that tweet — posted on Dec. 19, 2020 — as a clear-cut invitation. They almost immediately sprang into action, acquiring protective gear, setting up encrypted communications channels and, in one case, preparing heavily armed “quick reaction forces” to be staged outside Washington for the event.

Leaders of groups like the Proud Boys and the Three Percenter militia movement also started to whip up members with bellicose language as their private messaging channels were increasingly filled with plans to rush to Mr. Trump’s aid.

On Friday evening, Mr. Trump’s campaign announced what could be his first rally after an indictment: an event in Waco, Texas, where deadly clashes between federal officials and an extremist religious sect occurred 30 years ago around this time.

New York officials have been discussing security arrangements at the Manhattan Criminal Court in case of an indictment, according to people with knowledge of the planning, which was first reported by NBC News. Mr. Trump is expected to be charged in connection with the hush money payment his former fixer and lawyer, Michael D. Cohen, made to the porn star Stormy Daniels, who claimed to have had an affair with Mr. Trump.

Mr. Cohen made the $130,000 payment to Ms. Daniels to bury her story of the affair.

The payment came in the run-up to the 2016 presidential election, and Mr. Trump subsequently reimbursed Mr. Cohen. Prosecutors are expected to accuse Mr. Trump of overseeing the false recording of the reimbursements in his company’s internal records. The records falsely stated that the payments to Mr. Cohen were for “legal expenses.”

There have been several signals that charges may be imminent: The prosecutors gave Mr. Trump an opportunity to testify, a right afforded to people who will soon face indictment, and have questioned nearly every major player in the hush money saga in front of the grand jury.

Mr. Trump has denied all wrongdoing, as well as having had an affair with Ms. Daniels.

Any arrest and processing of Mr. Trump would probably combine the routine steps that every defendant experiences — fingerprinting, photographing — with the pomp accorded to a former president, whose every move is attended by the U.S. Secret Service.

It is unclear what kinds of accommodations Mr. Trump would receive. It is standard for defendants arrested on felony charges to be handcuffed, but an exception could be made. As he awaits his court appearance, it is possible that for security reasons, he will be detained in an interview room or another confined area, rather than in a holding cell. And after Mr. Trump is arraigned, he will almost certainly be released without spending any time behind bars, because the indictment is likely to contain only nonviolent felony charges.

Early Saturday morning, there was little evidence that Mr. Trump’s new demand for protests had been embraced by extremist groups.

But Ali Alexander, a prominent organizer of the “Stop the Steal” rallies following the 2020 election, reposted a message on his Telegram channel on Saturday suggesting that he supported a mass demonstration to protect Mr. Trump.

“Previously, I had said if Trump was arrested or under the threat of a perp walk, 100,000 patriots should shut down all routes to Mar-a-Lago,” Mr. Alexander wrote. “Now I’m retired. I’ll pray for him though!”

Without the platform provided by the White House or the machinery of a large political campaign, it is unclear how many people Mr. Trump is able to reach, let alone mobilize, via Truth Social.

And it remained unclear whether he would repeat his call for action or increase the stakes with more aggressive language. But his political allies made plain this week that they were preparing for a political war on Mr. Bragg.

For months Mr. Trump has been attacking Mr. Bragg, who is Black, as “racist.” Mr. Bragg won a conviction for tax fraud against the Trump Organization last year, though he did not charge Mr. Trump personally.

Some of Mr. Trump’s supporters responded of their own accord with violence after F.B.I. agents, acting on a search warrant, descended on Mar-a-Lago, his private club and residence in Florida, in August and carted away boxes of documents in an investigation into the former president’s handling of classified material.

Days after the search, an armed Ohio man who had posted online about his outrage over what happened at Mar-a-Lago tried to breach the F.B.I.’s field office outside Cincinnati. He was later killed in a standoff with local officers.

The unexpected Saturday morning salvo from the former president provided a preview of the kind of chaos that Mr. Bragg is likely to face if he proceeds with an indictment.

Mr. Bragg, a former federal prosecutor and deputy New York attorney general, has some history of prosecuting public officials. But he is unaccustomed to dealing with a figure as high-profile, erratic and pugilistic as the former president, and it is unclear how his office will deal with future outbursts from Mr. Trump.