Just more than a year after SB 1070’s divisive show-me-your papers provisions came into effect, Tucson marked the occasion with capsules of pepper spray.

Long-simmering tensions about how immigration laws ought to be enforced locally escalated Tuesday night into scuffles between Tucson police and Border Patrol agents, and anti-SB 1070 activists after a traffic stop turned into an immigration enforcement event.

Activists showed up quickly and by the dozen, surrounding a Border Patrol vehicle with a human chain and eventually being dispersed by police using pepper spray. Four people were taken into Border Patrol custody, and the driver received a civil traffic citation.

For activists, the protest was a reflection of their efforts to increase the visibility of a string of detentions and deportations they felt had mostly gone unnoticed but had been dramatically and painfully affecting immigrant families. They say that crowds could continue to appear at traffic stops leading to Border Patrol referrals unless the city’s police protocols change.

Advocates contest policies on when police refer someone to the Border Patrol, whether passengers in a car that is pulled over should be referred when they have committed no criminal offense under state law, whether a car should be impounded as a result and whether the immigration status of victims and witnesses should be questioned.

For his part, Tucson Police Chief Roberto Villaseñor says he is simply following the law — a law that he himself opposes but is mandated to follow and enforce.

“I think this is an excellent example of what I was worried about” when SB 1070 passed into law in 2010, he said at a news conference Wednesday. He is concerned that enforcing a law more closely linking criminal and immigration enforcement could erode community trust in police officers and hinder investigations because victims and witnesses are afraid to talk to police.

At the same time, Villaseñor warns that the sudden appearance of crowds in such situations threatens to “heighten an officer’s sensitivity,” potentially making an already volatile situation worse.

“It intensifies a traffic stop,” he said. “We’re just doing our job.”

Activists who oppose the law and protest collaboration between local police departments and the Border Patrol organized a year ago into the Protection Network Coalition, and recently they have tried to draw more attention to the law’s effects.

On Tuesday night, that meant that about 100 activists and supporters showed up within minutes of a coalition member being pulled over because of his vehicle’s unlit license plate and Border Patrol being called.

The driver, Agustín Reyes, is a member of Corazón de Tucson, one of the Protection Network Coalition’s constituent groups, the coalition’s spokesman, Raúl Alcaraz Ochoa, said. When the officer who pulled him over contacted the Border Patrol, Reyes called other members, who spread the word in a presketched tree of calls and texts designed for quick action.

“Usually these situations happen very fast,” Alcaraz Ochoa said. “They take people right away.”

Tucson police’s account of events confirms how quickly the call initially unfolded.

Reyes was pulled over at 6:50 p.m. near the Southside Presbyterian Church, 317 W. 23rd St., a church known for its advocacy on behalf of immigrant groups for decades.

Villaseñor said Reyes was not targeted based on his race, as the officer who cited him was also Hispanic.

Reyes did not have a driver’s license, and a backup officer found that he had never had one, which is a misdemeanor offense under Arizona law that required police to impound the vehicle, Villaseñor said.

Other factors the officer took into account in evaluating whether to call the Border Patrol were Reyes’ limited English proficiency and that his passenger, Arturo Robles, also had no license or ID and no English proficiency, and neither would provide further information to police, department spokeswoman Sgt. Maria Hawke said.

At 7:09 p.m., the officer requested the Border Patrol, and agents arrived at 7:23 p.m., Villaseñor said.

As the officer was issuing Reyes a citation, the crowd grew “verbally aggressive,” chanting, calling the police pigs and telling them to get out of the neighborhood, while also locking arms in a circle around the Border Patrol, Villaseñor said.

The Border Patrol called for backup and tried to leave, with TPD providing little assistance until someone yelled, “They are taking someone else.”

At that point, Villaseñor said, the crowd became “physically aggressive,” pushing and grabbing officers, a point that activists strongly dispute. “It was unprovoked,” said the Rev. John Fife, a longtime activist who is retired from Southside Presbyterian.

Officers used pepper spray, shooting it at protesters’ feet, he said, and trying to get them onto the sidewalks. Tucson firefighters were called to treat those most affected by the spray.

Activists and police say they would like to resolve their differences through dialogue but are reluctant to cede ground, and tentative steps toward talks in past months have gone nowhere.

City Councilwoman Regina Romero has decided it is time to take up the issue again at a political level. “What I have heard happened and the video I’ve seen alarms me,” she said. She has requested a City Council study session on SB 1070 for Nov. 13.

So far, no Tucsonan has logged a complaint with the city’s independent police auditor, Liana Perez.

Contact reporter Perla Trevizo at ptrevizo@azstarnet.com or 573-4210. On Twitter: @Perla_Trevizo