About 200 Central American migrants who were planning to take part in a traditional "Viacrucis" protest have been hemmed in by Mexican immigration checkpoints and threats to detain them, an activist said Saturday.

For several years, migrants have carried wooden crosses in protest marches around Easter week to illustrate their suffering at the hands of criminals and corrupt police.

But the Rev. Alejandro Solalinde, who runs a shelter for migrants, said authorities threatened to arrest on human-trafficking charges the owners of buses rented to take the protesters from the town of Ixtepec in Oaxaca state to the capital. He said the protesters had planned to leave for Mexico City on Thursday.

Almost 200 other migrants have also piled up at the shelter in Ixtepec because they are afraid of being detained at immigration checkpoints set up near the town, leaving Solalinde and his volunteers to feed and house about 420 people. He called the situation "critical."

"I've never seen a city under siege like this," Solalinde said.

Mexico's National Immigration Institute said in a statement that it keeps its checkpoints at least 6 miles (10 kilomters) from Ixtepec and that its agents are unarmed and human rights observers are present in the area. It said the road checkpoints are needed to detect migrants in the country illegally as well as people smugglers and they help "protect the migrants' safety."

The protesters are calling for an end to immigration raids that have largely prevented them from the riding a freight train north toward the U.S. border.

Solalinde said raids begun last summer to stop hitchhiking on the train had forced migrants to use more dangerous routes. He said that while about 15 percent of migrants were victims of robbery, extortion, assault and other forms of abuse previously, about 90 percent are victimized now.

"Before, they could get through faster and look after each other" on the train, Solalinde said. "Before, a stretch that would take them 12 or 13 hours, now it takes them a month. They are taking a dozen longer, more dangerous routes, where they are robbed or extorted."