People surround a bus carrying immigration detainees near a federal courthouse in Tucson in October after blocking its entry into the courthouse's parking lot. (Gary M. Williams/AP)

Eight men in clothes dirtied by their desert journey rise from their seats and approach the bench.

They wear no belts or shoelaces. They are chained around the waist, shackled by their ankles, cuffed at the wrists. They pad across the gray carpet under the recessed lighting in the William D. Browning Special Proceedings Courtroom.

“Are you pleading guilty because you are guilty?” Judge Charles R. Pyle asks.

“Sí,” the men say in unison.

“All answer ‘yes,’ ” the interpreter repeats.

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In a few minutes, these men and women from Mexico and Central America who are convicted of illegally entering the United States will be taken to a lockup by U.S. marshals in blue windbreakers. Then there will be eight more immigrants, and eight after that, until the courtroom is empty and 70 people — the day’s quota — will have passed through this judicial assembly line in federal court.

This is Operation Streamline. Fast-track prosecutions. Group hearings. From arrest to jail — with sentences as long as six months — in as little as one day. And under the Senate version of the immigration bill being debated in Washington, the number of people passing through U.S. District Court in Tucson under Streamline would triple — to up to 210 per day.

Critics say that with such speedy proceedings, immigrants do not receive a thorough defense.

“I’m just appalled,” said Isabel Garcia, a criminal defense lawyer and immigration activist in Tucson. “This program is endangering our very justice system.”

Sixty percent of those the United States is deporting are labeled “criminals” — a total of 216,810 people in the last fiscal year — but for many of them, their crime has been crossing the border. Over the past several years, the government’s efforts to stiffen the consequences of entering the United States illegally have led to a jump in criminal prosecutions and jail time for offenders. Previously, repeat illegal border crossers would often be deported rather than sent to jail.

Those who support Operation Streamline find it not only efficient but humane. Since the defendants are generally accused of reentering the United States after being deported at least once — a potential felony — the plea deals in the Streamline court get them reduced charges and less jail time than they otherwise might receive.

“The system is working well and the system is fair,” Judge Bernardo Velasco, who presides over Streamline cases here, said in an interview. “When you enter illegally you’re a criminal. You may not be a big criminal, but you’re a criminal. You can say ‘these poor people’ and all this other stuff, but they’re still criminals.”

Streamline defense attorneys, who often handle four to five defendants per day, meet with each one for up to 30 minutes in the morning. They present the government’s deal: Plead guilty to the misdemeanor of illegally entering the country and receive up to 180 days in jail, or plead not guilty and face felony charges and a sentence of between two and 20 years in prison, depending on one’s criminal record. Afterward, a representative from the Mexican consulate talks to the group — including immigrants from other countries — about the consequences of their decision.

Opponents argue that with such speedy proceedings, attorneys do not prepare a proper defense for their clients. A University of Arizona study last year found that 40 percent of the defendants surveyed said they were instructed by their lawyers to not fight the charges.

Potential defenses — such as being a juvenile or unfit to stand trial, or being eligible for citizenship or asylum — slip through the cracks, according to Heather Williams, a federal public defender who testified before a congressional committee six months after Streamline started in Tucson.

As the immigration debate in Washington has intensified, so has public criticism of Streamline, which Customs and Border Protection called an “important prosecutorial consequence” used to deter undocumented immigrants from entering the United States.

In October, police arrested at least 18 protesters who lashed themselves to the wheels of two buses carrying immigrants to their Streamline hearings and blocked an entrance to the courthouse parking lot. One of those arrested, Steve Johnston, a ­68-year-old activist, grew up in Alabama and worked in the civil rights movement before moving to Arizona.

“I think we will look back on this the same way my children look back on the ’50s and ’60s,” he said. “How could our government have treated our neighbors so poorly?”

Those who defend the immigrants argue that there is little to be done for them besides push for the shortest sentence.

“We have enough time,” said Richard Bacal, a private-practice lawyer who has worked with Streamline clients in Tucson since the program began in 2008. In 95 percent of the cases, he said, his clients have no defense.

Operation Streamline began in the Border Patrol’s Del Rio sector in West Texas in late 2005, as an effort to expedite cases at a time when the number of immigrants being picked up crossing the border was spiking. Since then, it has spread. By the end of fiscal 2012, more than 200,000 people were processed through the program — nearly half of all immigration prosecutions along the border during that period, according to a Congressional Research Service study from last year.

During one session last week, all 70 people accepted the government’s plea agreement. They received sentences of 30 to 180 days in jail.

Sometimes, the migrants ask to say a few words. On Jan. 21, a Mexican man in one of the groups addressed Judge Jacqueline Rateau after he pleaded guilty.

“I would like to say, I didn’t really want to come to the United States. I had a little shop over there in Michoacan. The mafia told me I had to pay them 500 pesos a month, and I didn’t have that much money to pay them,” he said, referring to a sum of about $37. “I had to leave Michoacan because they threatened me.”

Given the rampant violence in that part of Mexico, the man could have made an asylum claim, other lawyers said.

“I’m sorry you found yourself in that situation,” Rateau told the man. “And I hope you’re safe when you go back home.”

On Jan. 29, another shackled man requested that his jail sentence be reduced to 30 days.

“The reason being my family, my wife, my mother, my siblings, rely on me financially,” he told Velasco.

Velasco said he didn’t have the authority to reduce the sentence.

“Furthermore, I can assure you that if you look around in this courtroom, that everybody I have addressed today, and everybody I’m going to address the rest of this afternoon, more likely than not, is here because of economic considerations,” he said.

“Nevertheless, you violated the law. You know it. And you can either accept the sentence or enter a plea of not guilty and request a trial. Do you understand that?”