Hundreds of migrants from around a dozen countries left from Mexico's southern border on foot Sunday, as they attempt to make it to the U.S. border.

Some of the members of the group said they hoped to make it to the U.S. border before elections are held in November, because they fear if Donald Trump wins, he will follow through on a promise to close the border to asylum-seekers.

"We are running the risk that permits (to cross the border) might be blocked," said Miguel Salazar, a migrant from El Salvador. He feared that a new Trump administration might stop granting appointments to migrants through CBP One, an app used by asylum-seekers to enter the U.S. legally — by getting appointments at U.S. border posts, where they make their cases to officials.

The app only works once migrants reach Mexico City, or states in northern Mexico.

"Everyone wants to use that route" said Salazar, 37.

The group left Sunday from the southern Mexican town of Ciudad Hidalgo, which is next to a river that marks Mexico's border with Guatemala.

Some said they had been waiting in Ciudad Hidalgo for weeks for permits to travel to towns further to the north.

Migrants trying to pass through Mexico in recent years have organized large groups to try to reduce the risk of being attacked by gangs or stopped by Mexican immigration officials as they travel. But the caravans tend to break up in southern Mexico, as people get tired of walking for hundreds of miles (kilometers).

Recently, Mexico has also made it more difficult for migrants to reach the U.S. border on buses and trains.

Travel permits are rarely awarded to migrants who enter the country without visas and thousands of migrants have been detained by immigration officers at checkpoints in the center and north of Mexico and bused back to towns deep in the south of the country.

Oswaldo Reyna, a 55-year-old Cuban migrant, crossed from Guatemala into Mexico 45 days ago and waited in Ciudad Hidalgo to join the new caravan announced on social media.

He criticized Trump's recent comments about migrants and how they are trying to "invade" the United States.

"We are not delinquents," he said. "We are hard-working people who have left our country to get ahead in life, because in our homeland we are suffering from many needs."