Thousands of migrants set off from southern Mexico last week in one of the largest caravans seeking to reach the United States in recent years. The mass movement coincided with a recent meeting in Los Angeles, of leaders from the Western Hemisphere, where migration was a key focus.

Though migrant caravans have become a common phenomenon and are usually broken up by the authorities long before they reach the U.S. southern border, the latest march by some 6,000 people walking along Mexican highways has drawn significant international attention.

Many of the migrants came from Venezuela and had already trekked hundreds of miles through jungles and across multiple borders before arriving in Mexico. Once in Mexico, a migrant is usually required to stay in the southern city of Tapachula until the Mexican authorities grant a humanitarian visa to travel farther, a process that can take months.

“Tapachula has become a giant migrant jail,” said Luis García Villagrán, a spokesman for the caravan. “The Mexican authorities have a knot, a bureaucratic fence, a bureaucratic wall, obviously under pressure from the United States.”

Rather than languish in Tapachula, some migrants either pay human traffickers, many of whom have links to organized crime, or bribe immigration officials to speed up the process, Mr. García said in a phone interview.

Still others try to bypass the Mexican visa process and join the groups heading north, he said, believing that their large numbers will make it more difficult for the Mexican authorities to halt their progress.

A spokeswoman for Mexico’s National Institute for Migration said efforts were being made to provide migrants with legal paperwork in Tapachula.

“A good part of those who make up the caravan already have documentation,” said the spokeswoman, Natalia Gómez Quintero.

Still, Mexico’s National Guard, as shown in the photo below, is often dispatched to stem the flow of migrants north.

Stories of migrant mistreatment are widespread. A report by Human Rights Watch released last week found that “migrants and asylum seekers who enter Mexico through its southern border face abuses and struggle to obtain protection or legal status.”

Last year, Mexico apprehended more than 300,000 migrants — the highest number on record, according to Human Rights Watch, while more than 130,000 people have applied for asylum in the country. Such numbers have “overwhelmed” Mexico’s asylum system, the report said.

The presence of many Venezuelans in the caravan follows a shift in Mexico’s policy toward migrants from the South American nation, which has been consumed by political and economic crises. Since January, Venezuelans have needed visas to enter Mexico, a rule that many try to circumvent by crossing in large groups at land borders rather than flying.

Below, Rusbeli Martínez pushed a shopping cart alongside her son and other family members. After leaving Venezuela years ago, the family had been living in Colombia, which is home to roughly 1.7 million Venezuelan migrants. But in Colombia, she said, they found a harsh reception and little work.

“We lived in an area with a lot of crime — they threatened us that we should leave,” Ms. Martínez said. “Otherwise they would burn down the house.”

Many Venezuelans seeking a better existence have taken a difficult route over land, including traversing on foot the Darién Gap, a treacherous, roadless stretch of jungle in eastern Panama and northwestern Colombia. In the first five months of the year, more than 32,000 migrants, including over 16,000 Venezuelans, have made the crossing, according to Panama’s National Migration Service.

Eduardo Colmenares Pérez, a Venezuelan migrant who crossed the gap with his son and pregnant wife, said bandits had stolen all their belongings. “They left us without money, without food, without clothes, with nothing.”

Young men make up a large number of those in the caravan, but there are also many families with children. About 3,000 minors were traveling in the group, according to the United Nations Children’s Fund. Below, in a park in the town of Álvaro Obregón, a child played, while other young people sang.

Most of those in the caravan are poor and hoping for better opportunities in the United States. But some are also fleeing violence and persecution, including a group of L.G.B.T.Q. migrants who described the discrimination they faced in Venezuela and on the road.

Below, Maiquel Tejada, Yeider Rodríguez and Jesús Rangel gathered during a break in the caravan’s journey. “In Venezuela, and in the neighborhoods of Caracas, we’re not accepted,” said Mr. Rodríguez, center. “We have to repress ourselves, to pretend to be something we’re not.”

Others said they faced persecution for being outsiders. Yuliet Mora and her family left Venezuela and moved to Colombia and later Peru. But she said they were forced to leave because of xenophobia. In the first photo below, Ms. Mora sits under an improvised tent in Álvaro Obregón.

Roselys Guetiérrez and María Gómez, in the second photo below, are Venezuelans who used to live in Colombia, but left after they said they were assaulted for holding hands on the street in Bogotá.

“We decided to come through the jungle — it was pretty tough,” Ms. Gutiérrez said. “I’m pretty traumatized because of everything I lived through in the jungle, everything we lived through. But thanks to God I’m here hoping for something better.”

Some migrants decided to leave the caravan after Mexican immigration officials in the town of Huixtla in Chiapas state gave them temporary permits that allow them to freely transit the country toward the border for 30 days, according to Mr. García, the caravan spokesman. Other migrants decided to drop off the caravan entirely, exhausted by a trek that usually means walking miles every day, often in blistering sunshine or torrential rains.

Mexico is fraught with danger, particularly from organized criminal groups that are known to kidnap migrants and hold them for ransom, often paid by relatives in the United States. The caravan offers some safety in numbers, but the Mexican authorities have been known to disperse caravans by force.

Below, Venezuelan migrants stood on the roof of an immigration detention center in Tapachula following an uprising that migrants said was caused by poor sanitary conditions, a lack of food, overcrowding and delays in migration and asylum processing.

“We’re not criminals,” said one migrant, Valentina Alfonso, left, in the second photo below. She said her uncle had been detained by the Mexican authorities for several days. “We’re professionals, we have our careers, our studies,’’ Ms. Alfonso said. “This is inhumane.”

With temperatures that can reach as high as 100 degrees, the caravan usually sets off long before dawn. Below, a Venezuelan migrant pushed another migrant in a wheelchair as the caravan traveled through the night.

Mr. Colmenares, who had been in Mexico for five days after traversing the Darién Gap, has often had to rely on the generosity of fellow migrants for food.

“I feel enraged, impotent, because I had to abandon my country,” he said.

Despite the hardships, Mr. Colmares said he was thinking only of the road ahead. “What motivates me to keep walking is to search for my American dream,” he said. “To give my son a better future.”

Bryan Avelar contributed reporting.