Updated

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 about two hours 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. Demonstrators blocked city hall and a main avenue in the northern industrial city of Monterrey and roads in 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 allegedly 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 since then, 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 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.

Still, violence continued Tuesday in Ciudad Juarez, where three police officers — including the operations director of the city police — were shot to death by unidentified assailants on a street near the U.S. consulate. Another officer was wounded.

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.

Demonstrators also blocked bridges from Reynosa, forcing some nearby businesses to close, including one pharmacy that shut its doors for about two hours.

Asked if she believed the protesters were paid by drug traffickers, as the government has alleged, pharmacist Cristina Cazares said that while the news media does not report it, "everybody says that."

She said the protest ended without incident around the time that news of a gunbattle reached the bridge.

The federal Public Safety Department said the battle took place in Reynosa between federal police and a group of gunmen and that soldiers joined the fight when police asked for help. Five gunmen were killed and seven police officers were wounded, one seriously, the department said in a statement. Seven suspected assailants were detained.