Thousands of migrants imprisoned in southern Mexico for months joined a new caravan in protest on Sunday, heading for the capital to expedite their applications for asylum in the United States.

The migrants, predominantly Venezuelans, began their march north at Tapachula, a city on Guatemala’s border whose prison facilities have been overwhelmed by their large numbers. Some estimated that they would arrive in Mexico City in roughly ten days.

Fleeing violence and poverty in Central America, thousands of migrants walk together for safety to Mexico each year, crossing several states in hopes of finding a legal route into the United States.

The new caravan has about 3,000 migrants, some from China and other Asian countries, Tapachula authorities estimated.

“We joined the caravan to be safer and not be detained,” said Yoani, a Venezuelan migrant, who only gave his first name, by phone.

Out of money, he said his family was hoping to speed up the legal process needed for onward travel in Mexico City.

Irineo Mujica, director of the non-profit Pueblos Sin Fronteras (People Without Borders), said the new caravan arose as a form of protest by migrants demanding the closure of detention centers.

Last month, about 40 people died in a fire in one such center in Ciudad Juarez in northern Mexico.