Last year in New Jersey, federal immigration officers took more than 1,300 undocumented migrants into custody. That figure was roughly 300 more than in 2023.

But on Thursday, less than a week into President Trump’s second term, the arrests of three people at a fish distribution warehouse in Newark appeared to tap a well of pent-up fear about mass deportations in a region teeming with immigrants.

The streets around the warehouse filled early Friday with television crews. Newark’s mayor held a news conference to decry the methods used by Immigration and Customs Enforcement officials as unconstitutional and blamed Mr. Trump, who campaigned on a promise to initiate the “largest deportation program in American history.”

Whether Thursday’s arrests in Newark’s Ironbound neighborhood were part of a new crackdown, or fairly typical of ICE enforcement actions in the city in recent years, was not immediately clear. Immigration arrests in the city are common. Last month, under former President Joseph R. Biden Jr., ICE officers based in Newark announced 33 arrests to little public notice. And ICE officials did not reply to several requests for comment.

But the enforcement activity left immigrants across the region on edge. There were reports of ICE officers knocking on doors in Vineland, in New Jersey’s southern agricultural region, which is heavily dependent on migrant labor. On Long Island, immigrant rights activists said they were busy fielding reports of “ramped-up” activity by ICE officers. And a police captain in Ossining, N.Y., Brendan Donohue, warned that rumors often multiply more quickly than facts.

“Fear spreads very quickly, and even just the suggestion that ICE could come here turns into a ‘ICE was here’ kind of a situation,” Captain Donohue said. “These things can snowball, of course.”

Merchants in Newark who run body shops and cafes near the fish distribution center, Ocean Seafood Depot, said Thursday’s midday raid was unusual for the industrial neighborhood, which is dotted with two-story homes and some of the city’s best restaurants.

Newark’s mayor, Ras J. Baraka, a Democrat who is running for governor, warned that the city intended to defend its residents.

“If he thinks that we’re just going to go to jail quietly,” Mr. Baraka said of Mr. Trump, “he’s got another thing coming.”

Immigration officers entered legally through a fish store at the front of the facility. But Mr. Baraka said that they had proceeded, without presenting a warrant, into a large nonpublic warehouse where workers pack fish and load it onto delivery trucks.

He said ICE officials had also challenged the validity of a military ID presented by a U.S. citizen who works at the warehouse and was questioned during the raid. Mr. Baraka urged workers and their employers to become familiar with their rights — before ICE officers show up.

“We can disagree about whether you support mass deportation or not,” Mr. Baraka said. “But what we must agree on is — the thing that separates this country from many other countries around the world — is the Constitution.”

“Everyone has a right to due process,” he added, “and no one can go around these laws.”

Amy Torres, executive director of the New Jersey Alliance for Immigrant Justice, said she had raced to the warehouse after receiving reports of a raid before lunchtime on Thursday.

“They were heavily armed,” she said of the uniformed officers who conducted the search.

“They were blocking off entrances and exits. They were scrambling up delivery ramps. They were banging down bathroom doors to make sure no one was hiding inside,” she added.

All but a handful of the roughly 80 people who work at the warehouse abruptly left for the day, fearing a repeat visit by the enforcement agency, Ms. Torres said.

The effect of the enforcement action remained palpable on Friday. Barbershops were empty along a normally bustling commercial corridor near the seafood company. Customers were scarce at a cafe that its owner said routinely fills each morning with warehouse workers who come in to buy coffee before their shifts.

A Newark councilman who lives in the area, Michael Silva, said he, too, had noticed an immediate change.

He said he typically wakes each morning at 4:45 a.m. to the sound of his next-door neighbor opening a gate to leave for work.

“This morning, I didn’t hear that gate,” said Mr. Silva, the son of Portuguese immigrants. “He told me that he was scared to go to work.”

Jessica Greenberg, the legal director at CARECEN-NY, an organization that works with immigrant communities on Long Island, said that alarm about Mr. Trump’s immigration policies had intensified over the last week.

“They are going after people that were considered ‘low-hanging fruit’ in past administrations,” Ms. Greenberg said, adding, “We’ve been on the phone with individuals while ICE has been banging on their door or shortly after ICE has left.”

ICE arrests are hardly novel in the region. In December, while Mr. Biden was still in office, ICE officers based in Newark conducted what the agency called a “weeklong, targeted, surge operation.”

Still, immigrant rights leaders have been holding events designed to instruct documented and undocumented residents on their rights in anticipation of a broad crackdown by Mr. Trump.

New Jersey education officials also released guidance this week to school leaders, offering instructions on what to do if immigration officials show up at public schools. The instructions came in response to Mr. Trump’s Tuesday announcement that ICE and Homeland Security officers would no longer be barred from detaining people at schools or churches, so-called sensitive locations that since 2011 had been considered safe spaces.

Rui Lorenço works at a car repair shop in Newark’s Ironbound neighborhood, which is home to a large number of Portuguese, Brazilian and Ecuadorean residents. He said he had noticed heightened panic on social media over the last week.

Mr. Lorenço, who moved to the United States about five years ago from Lisbon, said he supported clearer rules on immigration, but not what he described as “hate speech” spread by Mr. Trump and his supporters.

“This is a country made of immigrants,” Mr. Lorenço said. “If they come to take people away that are just working, that’s concerning.”

Larissa Cardoso, 22, emigrated to the United States from Brazil about a year ago. She said she was afraid about what a stricter immigration policy could mean for her and her friends in the days ahead.

“I always dreamed to come here, and I try to do things right,” said Ms. Cardoso, a waitress and bartender in a popular Ironbound restaurant who has been working to gain legal immigration status.

“People come here because they literally want to change their lives,” she said. “With what’s happening now — their lives could now stop.”

Hurubie Meko and Lola Fadulu contributed reporting.