LOS ANGELES — Safir Wazed, a graduate student born in Bangladesh and raised in California, struggled to focus on his studies. Evelin Salgado, born in Mexico and raised in Tennessee, was ending plans to buy a house and wondering what would happen to her teaching job.

And Basilisa Alonso did what thousands of other so-called Dreamers did on Tuesday: She marched in the streets to make her plight known.

“I’m willing to take the risk for my family and for all the other DACA people out there,” Ms. Alonso said, referring to Deferred Action for Childhood Arrivals, the Obama-era program that she was marching to save. Minutes later, a New York City police officer moved in and placed her hand behind her back with plastic restraints. She was among several dozen people arrested after they blocked an intersection near Trump Tower at various times on Tuesday.

About 800,000 undocumented young adults like them had endured weeks of nail-biting tension over the fate of DACA, which for the last five years has enabled them to legally live and work in the United States.

On Tuesday, Attorney General Jeff Sessions confirmed days of speculationthat the Trump administration would end DACA in six months, leaving Congress to come up with a legislative solution to replace it. The announcement, fulfilling a campaign pledge of President Trump, immediately threw into question the future of everyone who signed up under the program.

“I have been blessed with all the opportunities that DACA brought to my life,” said Ms. Salgado, 23, who is now a teacher and was the first person in her family to graduate from college.

Since receiving DACA status, Mr. Wazed, 27, has held a job and bought a car and a condominium. He is now a graduate student at the University of Southern California. “Am I supposed to plan to reset my life in six months?” he asked.

“This isn’t over,” Mr. Wazed said, “and we’re not going to be pushed out of our country in six months.”

What ‘Dreamers’ Gained From DACA, and Stand to Lose

Indeed, the decision has immediately reawakened a protest movement that helped lead to the program’s creation.

About a decade ago, leaders of undocumented young immigrants used activities like sit-ins and demonstrations to share their cause with the American public, until they reached the White House. Those efforts were repeated Tuesday in cities across the country, from New York to Washington to Denver to Los Angeles, now greatly amplified by social media and institutions — corporate boardrooms, colleges and religious organizations — that touch virtually all corners of society.

In a statement, the nation’s Roman Catholic bishops called the decision “reprehensible.” The Rev. Samuel Rodriguez, president of the National Hispanic Christian Leadership Conference, an evangelical group, announced it will move staff to Washington and fly in hundreds of pastors in October to put pressure on Congress. “We do not intend on letting a single member of Congress have a good night’s rest until they guarantee our young people can rest easy,” he said.

The broad reaction illustrated not only how successful the Dreamers and their advocates had been in persuading the Obama administration to create the program, but how, after five years of living legally in the United States, they had become fully integrated into American colleges, workplaces and civic life. DACA gave immigrants who had been brought to the country illegally as children a reprieve from deportation and a work permit if they met certain conditions.

A survey released recently by the left-leaning Center for American Progress that found at least 72 percent of the top 25 Fortune 500 companies employ DACA recipients. Joining it on Tuesday was the U.S. Chamber of Commerce, a business advocacy group, which called the administration’s decision “contrary to fundamental American principles and the best interests of our country.” Prominent executives implored the president and Congress in an open letter to keep DACA, saying the recipients were vital to their companies and to the economy.

So with the administration’s decision looming, many Dreamers recently began contemplating a double loss, of their legal protection and the lives and careers they had begun with that protection.

Last week, amid the uncertainty over the program, Ms. Salgado told her real estate agent that she had to scrap plans to buy a four-bedroom house in Fairview, Tenn., for her parents, with whom she lives.

In New York, during the march down Fifth Avenue, some stopped and watched on their smartphones as Mr. Sessions made the news official. “We pay our taxes, follow the laws,” said Dayana Arrue, 22, as she sobbed beneath her Ray-Ban sunglasses. She came from El Salvador when she was 6, is now a senior at Rutgers University, and was planning to go to graduate school for geoscience engineering. “All that talent that the U.S. is missing out on, it’s unbelievable,” she said. “It kind of all ends.”

Protests continued throughout the day, and the New York Police Department said it had arrested more than 45 people, including some with DACA status. But they would not be fingerprinted if they followed court instructions, according to the police commissioner, James P. O’Neill.

That could protect them from being discovered by federal immigration authorities. Still, Mayor Bill de Blasio cautioned them during a news conference: “Obviously if someone is a DACA recipient, I would urge them to be careful about anything like civil disobedience.”

On the campaign trail, Mr. Trump pledged to “immediately terminate” the program. Once elected, he softened his stance, praising Dreamers as “absolutely incredible kids” who deserve compassion.

Feeling reassured, tens of thousands of young immigrants filed new or renewal applications. But the program’s fate became uncertain after a group of conservative state attorneys general, led by Ken Paxton of Texas, threatened to sue the administration unless it began to dismantle the program by Tuesday.

Most DACA beneficiaries live in one of five states — California, Florida, Illinois, New York or Texas — and the largest share are from Mexico or Central America. But thousands hail from South Korea, the Philippines or India, among many other countries, and arrived with visas that their families overstayed.

“In the coming days, weeks and months, many Americans will find out — for the first time — that they know someone with DACA,” said Katharine Gin, executive director of Educators for Fair Consideration, a nonprofit in San Francisco that works with Dreamers. “Maybe it’s their child’s teacher, or the nurse who takes care of their mother, or the young person they always sit next to at church.”

Still, some Dreamers noted there could be a silver lining in Mr. Trump’s decision. The program had always been on infirm legal footing, as conservatives argued that former President Barack Obama did not have the power to create it without Congress. With the president now urging Congress to do something to replace DACA, it was not impossible to dream that a permanent solution was at hand.

“Ending DACA may be the only solution to Congress acting rapidly,” said Monica Lazaro, a DACA recipient in Miami who is set to start a research job at a veterans’ hospital later this month. “We will push Congress, and especially Paul Ryan. This is not the end, it is just the beginning.”

As Mr. Sessions spoke, Marcela Zhou, a Dreamer and third-year medical student at the University of California, Los Angeles, paused between patients she was seeing at a clinic to watch the announcement.

“I have been anticipating this moment, but it is hard to fully mentally prepare for it,” Ms. Zhou said.

She added hopefully: “The future remains uncertain, but I am confident I will become a doctor one day. The road may be longer and bumpier, but we will get there.”

Read more at Worried and Frustrated, ‘Dreamers’ Say They Won’t Give Up.