Unlawful crossings along the Southern border have reached levels not seen for several months, straining government resources and taxing some local communities where large numbers of migrants have been released from federal custody.

There were more than 8,000 arrests on Monday, according to Brandon Judd, the head of the union that represents Border Patrol agents. Such high numbers haven’t been seen since a surge in early May brought the daily number to nearly 10,000, and they are far higher than in mid-April, when there were about 4,900 illegal crossings a day.

The effects of the increasing numbers ripple across the country, as communities on the border and others far from it find themselves scrambling to support migrants released from federal custody.

“Right now we are seeing a surge,” said Ruben Garcia, who oversees a network of shelters in El Paso, across the border from Ciudad Juárez, Mexico. “We have a significant increase in the number of people crossing.”

The recent influx in unlawful crossings could present challenges for President Biden, whose administration has sought to keep the Southern border from fueling Republican narratives about immigration policy, particularly before the 2024 presidential election.

During President Biden’s time in office, the number of illegal crossings has reached notable highs, exceeding levels seen during a prepandemic influx in 2019 during the Trump administration. But crossings on the Southern border declined sharply for about six weeks in May and June after the end of a public health measure put in place during the pandemic. Known as Title 42, the rule resulted in the swift expulsion of migrants who had crossed the border illegally, even if they were seeking asylum.

Officials had expected a spike in illegal crossings after the termination of Title 42, but the increase came days before, reaching about 9,500 a day in the week before Title 42 ended.

The relative quiet that followed did not hold.

“I never believed the decline in unlawful border crossings would last, because there were already tens of thousands of people in northern Mexico and many more behind them coming up through the Darién Gap,” said Theresa Cardinal Brown, senior adviser for immigration and border policy at the Bipartisan Policy Center.

Last year, a record of nearly 250,000 people traversed the Darién Gap, a jungle straddling Colombia and Panama, in an attempt to make it to the United States. This year, despite efforts by the United States to curb the flow, that number has risen to 360,000 as of Sept. 10, according to Panamanian authorities.

The administration said the decline in unlawful crossings in May and June was driven by new enforcement measures and new legal pathways for people to come to the United States.

Officials attribute the recent influx to several factors, including the long waits that come with the new Biden administration pathways and misinformation spread by the Mexican cartels that traffic drugs and smuggle migrants.

Customs and Border Protection, which tracks border crossings, did not confirm the recent numbers, information that is customarily made public about three weeks after being compiled.

In a statement, a C.B.P. spokeswoman, Erin Waters, said that the agency expected to see “fluctuations” and was “working to decompress areas along the Southwest border.”

She said that those taken into custody were being placed in immigration enforcement proceedings and that anyone without a legal basis to stay would be removed.

Starting in July, many people, including families, waiting for an appointment at a port of entry or through a humanitarian parole program, have decided to take their chances and cross the border illegally, people who work with asylum seekers and in migrant shelters said. Even as federal officials signal that there are consequences for illegal crossings, migrants who are given permission to stay in the country temporarily often tell family and friends in their home countries that they made it to the U.S. successfully. Such messages can encourage other migrants to take an often dangerous journey to the United States.

This influx has strained the capacity of many border facilities where migrants are held for processing by the Border Patrol. And Immigration and Customs Enforcement detention centers, where many single adults are sent, are running out of beds. When shelters cannot accommodate migrants, authorities start to release them into communities.

“The Border Patrol essentially is releasing people as they process them to decompress their facilities,” Diego Piña Lopez, director of the Casa Alitas shelter network in Tucson, said. “It is leading to street releases all over the place.”

In southern Arizona over the past week, mayors and local officials said that after processing dozens of migrants, border officials released them in small border towns, dropping them by a Catholic church in Douglas or a supermarket in Bisbee with no means.

“We had 32 of them yesterday that were dumped off at 3 in the afternoon, and there were no buses,” Mayor Ken Budge of Bisbee said.

Casa Alitas, which operates five shelters in the Tucson area, has been accommodating 1,500 people each night, up from 800 two weeks ago.

In San Diego, border officials have been dropping hundreds of migrants a day at transit hubs, as migrant shelters in the area reached capacity. Volunteers have tried to provide basic needs, including food, water and assistance with onward travel, but shelter space elsewhere is limited as well.

“The situation is not sustainable for the community organizations trying to meet the humanitarian needs of migrants in these border areas,” Pedro Rios, director of the U.S.-Mexico border program for the American Friends Service Committee, said.

In El Paso, a cargo bridge between Mexico and the United States has been closed for several days, because customs personnel were diverted to assist Border Patrol agents with the processing of migrants who have been apprehended.

On Sept. 18, agents in the El Paso sector encountered 1,609 migrants, according to official data obtained by The Times, up from 1,158 on Sept. 7 and 761 on June 9.

After crossing onto U.S. soil, most migrants turn themselves in to Border Patrol agents, with plans to apply for asylum, instead of sneaking into the country and trying to evade detection.

Jack Healy in Phoenix, Reyes Mata, III, in El Paso, and Julie Turkewitz in Bogota contributed reporting.