WASHINGTON — President Trump signed an order on Wednesday to start building a border wall with Mexico and was planning to indefinitely block Syrian refugees from entering the United States and to institute a temporary halt on all refugees from the rest of the world.

The refugee policies are part of an executive order he is expected to issue as soon as Thursday, according to an eight-page document, a copy of which was obtained by The New York Times.

The order would require tougher vetting of foreigners fleeing persecution and place a monthlong ban on allowing any person into the United States from Iraq, Syria, Iran, Sudan, Libya, Somalia or Yemen. Refugee admissions would be halted for 120 days while a review of screening procedures is completed. When it resumes, the program would be far smaller, with the total number of refugees resettled in the United States this year more than halved, to 50,000 from 110,000.

White House officials declined to comment on the forthcoming plan, which emerged as Mr. Trump announced the construction of his long-promised Mexican border wall and aggressive new measures intended to crack down on undocumented immigrants inside the United States.

Through a pair of executive orders he signed at the headquarters of the Department of Homeland Security, Mr. Trump was laying out a new vision for fortifying the nation’s borders and sharply increasing efforts to round up and remove some of the estimated 11 million undocumented immigrants living in the United States — including by enlisting state and local officials to track and apprehend them.

“Federal agencies are going to unapologetically enforce the law — no ifs, ands or buts,” said Sean Spicer, the White House press secretary. “The American people get the final say who can and cannot enter our nation.”

The plans were a stark break with former President Barack Obama’s approach and what was once a bipartisan consensus to devise a path to citizenship for some of the nation’s undocumented immigrants. Mr. Trump, whose campaign rallies featured chants of “build the wall,” has instead described many undocumented immigrants as criminals who must be found and forcibly removed from the United States.

“They’re setting out to unleash this deportation force on steroids, and local police will be able to run wild, so we’re tremendously concerned about the impact that could have on immigrants and families across the country,” said Joanne Lin, senior legislative counsel at the American Civil Liberties Union. “After today’s announcement, the fear quotient is going to go up exponentially.”

Until the refugee orders are signed, it is possible the language could change.

The order on refugees is also in line with a Muslim ban that Mr. Trump proposed during the campaign, though it does not single out any particular religion in its text. It orders the secretary of state and the secretary of homeland security to prioritize those who are persecuted members of religious minorities, effectively ensuring that Christians living in predominantly Muslim countries would be at the top of the list.

“In order to protect Americans,” the order states, “we must ensure that those admitted to this country do not bear hostile attitudes toward our country and its founding principles.”

It says that for the time being, admitting anyone from Iraq, Syria, Iran, Sudan, Libya, Somalia or Yemen is “detrimental to the interests of the United States.”