Hundreds of people blocked bridges to the United States in three border cities Tuesday, demanding the army leave in another challenge for the Mexican government as it struggles to quell escalating drug violence.

The protests in Ciudad Juarez blocked traffic for at least an hour across three bridges connecting the city to El Paso, Texas. Similar protests broke out on bridges in the border cities of Nuevo Laredo and Reynosa, while demonstrators blocked roads in the northern industrial city of Monterrey and the Gulf state of Veracruz.

It was the largest display of discontent against the army's role in an anti-drug crackdown since President Felipe Calderon began deploying soldiers across the country two years ago to fight cartels. About 45,000 soldiers are now spread out across Mexico.

Government and army officials claimed that drug cartels organized similar protests in Monterrey earlier this month to undermine the crackdown. Federal officials had no immediate comment on Tuesday's protests.

Human rights activists say there are legitimate complaints about abuses by soldiers, including cases in which patrols opened fire on civilians at military checkpoints. But they say it is unclear who has been behind the demonstrations.

Calderon's offensive was initially widely popular among Mexicans hopeful for an end to relentless shootings, kidnappings and killings. But drug violence has only surged and become more violent since he took office, with drug gangs beheading rivals and attacking police nearly every day. More than 6,000 people were killed in drug violence last year.

Border towns have been transformed by the crackdown, with soldiers in ski masks regularly rumbling down the streets in large convoys.

About 30 people, mostly women and children, blocked the Paso del Norte bridge leading from Ciudad Juarez to downtown El Paso. They shouted "soldiers, get out!" as they stood in front of about 20 troops in green army pickup trucks. One person held a sign reading, "Get out of Juarez, thieving abusive soldiers."

Brenda Contreras, a 22-year-old mother of two, said the city has only become more dangerous since troops arrived. She said soldiers detained her 32-year-old husband during a raid at a car wash three months ago, and she has not heard from him despite filing a complaint with the Defense Department.

"I saw him on the floor and they wouldn't let me get near him. Where he is, only the army knows. What am I going to tell my daughters?" she said.

The protesters waved to soldiers and dispersed peacefully after about an hour, allowing a long line of cars to proceed across the bridge.

Not all city residents, however, are against the army's presence.

"Knowing that the soldiers are out there all over the city makes me feel that the city is more protected," said Cynthia de los Santos, 32, a secretary at a Ciudad Juarez law firm.

In Nuevo Laredo, 150 people blocked three bridges leading to Laredo, Texas, for several hours. Many were masked men, but some were women and children.

Protesters also blocked bridges leading to Texas from Reynosa, the town's mayor, Oscar Luebbert, told Mexico's Radio Formula.

Luebbert also said a shootout erupted Tuesday between federal police and armed men in Reynosa, and authorities were trying to confirm reports that several people were killed. He said the shootout appeared unrelated to the demonstrations.

"The situation is very critical," Luebbert said. "The whole population is very alarmed."

An official from the federal Public Safety Department said the shootout erupted when federal police came across a group of gunmen. Soldiers joined the fight when police asked for help, said the official, who was not authorized to give his name. He said at least four gunmen were killed and several police were injured.

In Monterrey, about 200 people protested in front of city hall, choking traffic for about an hour. The demonstrators — women, children and some men covering their faces with scarves and T-shirts — dispersed peacefully when police in riot gear arrived.

Several hours later, masked young men briefly blocked Monterrey's main avenue.

Protesters also blocked two highways in the Gulf coast state of Veracruz and one road outside the state capital, Jalapa.