President Biden on Wednesday shielded thousands of Palestinians in the United States from deportation for the next 18 months, using an obscure immigration authority as he faces mounting criticism over U.S. support for Israel in the Gaza war.

About 6,000 Palestinians are eligible for the reprieve under a program called Deferred Enforced Departure, which allows immigrants whose homelands are in crisis to remain in the United States and work legally.

In a memo obtained by The New York Times, Mr. Biden said that “many civilians remain in danger” in Gaza after the Oct. 7 attacks by Hamas.

“Therefore, I am directing the deferral of removal of certain Palestinians who are present in the United States,” he said.

The decision comes as Mr. Biden faces pressure over the war, particularly among Arab Americans who were once a reliable constituency for him. In recent weeks, pro-Palestinian groups have been demonstrating outside his campaign stops, chanting “Genocide Joe.”

While Mr. Biden’s criticism of the war has grown more forceful since the Oct. 7 attack, the United States has not signaled that it plans major policy changes such as putting conditions on billions of dollars in military aid to Israel.

Israel’s war against Hamas has killed more than 28,000 Palestinians, according to Gaza’s health ministry. Much of Gaza has been left in ruins as Israel bombards the territory in retaliation for the attacks on Oct. 7, when Hamas killed more than 1,200 people in Israel.

Abed Ayoub, the executive director of the American-Arab Anti-Discrimination Committee, praised the decision to exempt Palestinians from deportation.

“There is a desperate need for this,” he said. “We see the situation in Gaza and Palestine is not getting better, and this is something that is welcome, and we are glad to see it implemented. We hope other measures can come into place.”

There are some exemptions to Mr. Biden’s order. Palestinians who have been convicted of felonies or those “who are otherwise deemed to pose a public safety threat” would not be protected from deportation, Jake Sullivan, Mr. Biden’s national security adviser, said in a statement.

Some Republicans, meanwhile, have pushed for a crackdown on Palestinians. Representative Ryan Zinke of Montana, a former Trump administration official, introduced legislation in November that would have revoked visas from Palestinians and prevented them from receiving refugee status or asylum in the United States.

Mr. Biden’s decision to shield Palestinians from deportation has been in the works for some time. More than 100 staff members at the Department of Homeland Security signed an open letter to Alejandro N. Mayorkas, the homeland security secretary, in the fall, saying the agency should extend some protections to Palestinians.

Some congressional Democrats have also called on the administration to find a way to protect Palestinians in the United States.

“In light of ongoing armed conflict, Palestinians already in the United States should not be forced to return to the Palestinian territories, consistent with President Biden’s stated commitment to protecting Palestinian civilians,” they wrote in November in a letter, which Senators Richard J. Durbin of Illinois, Elizabeth Warren of Massachusetts and others signed.

The lawmakers said the population should be covered under Deferred Enforced Departure or a similar program known as Temporary Protected Status, which has been used to help people from Venezuela, Afghanistan, Ukraine and elsewhere. (Deferred Enforced Departure is currently being used to help people from Hong Kong and Liberia.)

Ahilan Arulanantham, a director of the Center for Immigration Law and Policy at the U.C.L.A. School of Law, said the short-term practical effect was the same under both programs.

“Every qualifying individual would have protection from deportation and the ability to obtain employment authorization,” he said.

But he cautioned that the longer-term differences could be significant. Palestinians could be more at risk of having the protections lapse in 18months because they are at the discretion of the president, Mr. Arulanantham said.

Temporary Protected Status, by contrast, requires agency officials at the Department of Homeland Security to assess the protections before they expire.

Earlier this month, Mr. Biden ordered financial and travel sanctions on four Israeli settlers accused of violent attacks on Palestinians in the West Bank. While the war is centered in Gaza, there is also growing violence in the West Bank, which Israel has occupied since 1967 and is home to more than 2.5 million Palestinians.