A block-sized park, darkened in morning shadow by the courthouse across Centre Street, erupted in a chaos of jeers, flags, whistles and the clanging of spoons against metal pots as protesters converged to defend or bash Tuesday’s famous defendant.

Hours before the arrival of Donald J. Trump at the Manhattan Criminal Courts Building, hundreds of people gathered in groups or stood alone beneath leafless trees, spreading banners or holding handmade signs on cardboard. The opposing groups were divided by police barricades. Intruders who infiltrated their opponents’ turf were quickly surrounded and shouted down or shoved.

Representative Marjorie Taylor Greene, a far-right Georgia Republican closely aligned with Mr. Trump, had promoted the rally at Collect Pond Park in the days before the arraignment that made him the first ex-president to face criminal charges.

Ms. Greene arrived and spoke through a megaphone, her words drowned out by protesters from both camps. After about five minutes, she was ushered out of the park by the police as protesters banged on the windows of the Ford Expedition in which she rode.

Representative George Santos, a Republican of Long Island, also visited the park, but he too left soon after, trailed by shouts from protesters about a history of lies that was uncovered only after he was elected to his first term. Asked if he planned to travel to Mr. Trump’s Florida estate, Mar-a-Lago, to support him later Tuesday, he said, “Guys, I don’t have a plane.”

Collect Pond Park, named for a long-ago reservoir, was part of the once notorious Five Points tenement slum and, in more recent years, was overrun by rats. On Tuesday, its 0.99 acres became the stage for outrage and spectacle, filling with shouts:

“Four more years!”

“U.S.A.!”

“We love Trump!”

One young man was dressed like the courthouse-bound former president. Another came disguised as Abraham Lincoln.

A woman from Ireland who lives in Queens, calling herself Marie Free — a fake last name to keep harassers away, she said — displayed a picture of Mr. Trump and, curiously, another of Dr. Martin Luther King Jr.

She said she thought the slain civil rights leader and the former president would have gotten along: “They really would have so much in common. They both love this country.”

With reporters and cameras thronging the park, many protesters seized on the chance to promote causes that had nothing to do with Mr. Trump’s arrest. A man walked past shouting an anti-trans slogan, and the woman with the picture of Dr. King joined him, yelling together: “two genders!”

Ozzie Hernandez, 58, of the Lower East Side, planted himself deep in opposing territory with a sign that read “Trump Felony” and shouted in support of Mr. Trump’s arrest. He was immediately flanked or surrounded by police officers — eight of them at one point — as pro-Trump protesters jeered him.

“They’re worried I’m going to provoke something,” Mr. Hernandez said. He reassured the officers: “They can call me whatever they want. I’m not here to hurt people — I’m New York, born and raised.”

He paused to allow a video journalist to clip a microphone to his shirt for an interview. A man in a rubber Trump mask and an orange prison jumpsuit walked past, stopping at a fence to grab the bars like a prison cell. Countless hands clutching smartphones were raised in his direction. He held the pose.

In contrast, pro-Trump protesters who ventured into the opposing camp appeared to face elevated harassment.

A contributor for the far-right network Newsmax and a cameraman entered the anti-Trump section and tried to interview people. They met a wall of protesters, who steadily pushed them back. One person pushed the reporters with a black umbrella.

The cameraman turned his rig off and warned the correspondent, Cara Castronuova, to be careful. “Just capture the story!” she replied. “Turn your camera on. Let’s do this!”

A police officer shouted “Get out of here! It’s not safe.” The reporters left.

A man wrapped in an American flag ventured into the anti-Trump ranks and was confronted by young men, their faces covered in black balaclavas. When they ordered him to leave, a bystander, Russ Meyer, 47, of Manhattan, tried to intervene.

“I’m not pro-Trump at all — I’m a Libertarian,” Mr. Meyer explained. “It’s a public park. You can’t threaten people.”

At times, the park seemed to be half protesters, half journalists or video bloggers, arriving from seemingly all over the world with camera crews or phones in tow. Several walked alone, narrating to their screens.

By midday, the shadow of the courthouse was gone, and the spring sunshine — the temperatures rose toward 70 degrees — threatened to turn the mostly white faces shades of pink. A woman passed out fliers promoting a claim that Mr. Trump, in a past life, was George Washington, making him both the nation’s first and 45th president.

Richard Fisher brought a sign with battery-powered lights that read “LOCK HIM UP,” and said he was pleased by the upbeat mood of those around him.

“Trump wanted to stir up all this anger and violence,” said Mr. Fisher, 66, of Columbus Circle in Manhattan. “And as you can see, it’s not working.”

When Mr. Trump’s motorcade arrived at the district attorney’s office shortly before 1:30 p.m. few at the park seemed to notice — the very object of all the defending or derision slipping past unseen. Many people read about his arrival after the fact, on their phones.

Mr. Trump pleaded not guilty to 34 counts of filing false business records and left the courthouse as he arrived, unobserved.

In the park, as the afternoon wore on, the police presence appeared to shrink, which worried Kate Irwin, 47, a self described “stage manager of the resistance” on the anti-Trump side.

"We need to watch out,” she said. “Right now is when it starts getting a little weird.”

Chelsia Rose Marcius contributed reporting.