The white Chicago police officer who shot black teenager Laquan McDonald 16 times in an encounter captured on dashcam video has been found guilty of second-degree murder.

After a second day of deliberation, the jury brought a verdict that eased anxieties about public disturbance in a city struggling to overcome racial tension between black communities and a predominantly white police force.

The verdict will be greeted as a measure of justice by the black community. As demonstrators gathered outside the court, listening to the verdict on cell phones, one woman said: “Thank you, Jesus.” There were cries of “Yes!”

But the verdict was inevitably divisive. Chris Southwood, the president of the Fraternal Order of Police, claimed the jury had been “duped” and said police had been “stabbed in the back” by “political operatives” in a “sham trial and shameful verdict”.

Police were put on 12-hour shifts to bolster available numbers by up to 4,000. However, officers will not be in riot gear, because the force is anxious not to send a message that they are ready for a fight, Supt Eddie Johnson told the Chicago Sun-Times.

The jury was instructed that first-degree murder would carry a sentence of 45 years to life. The sentence for second-degree murder ranged from probation to 20 years. Jason Van Dyke was also found guilty on 16 counts of aggravated battery with a firearm, which carries a mandatory minimum sentence of six years and a maximum of 30. He was found not guilty of official misconduct. He will be sentenced at a later date.

The shooting of McDonald happened in October 2014 but did not gain notoriety until video was released by court order in 2015, the same day officer Jason Van Dyke was charged.

Van Dyke faced one count of first-degree murder, one charge of official misconduct and 16 counts of aggravated battery, one for each shot fired. The jury was given the option of second-degree murder, which would indicate that the killing was not premeditated, but that Van Dyke showed obvious disregard for the consequences of his action.

The video showed Van Dyke’s car pull up as McDonald, 17, was striding down the middle of a two-way street, carrying a knife. Police had answered a 911 call after McDonald was reportedly seen attempting to break into a truck.

The jury heard that the first police officers on the scene kept a distance from McDonald as he walked. An autopsy would later show that McDonald, who was carrying a three-inch bladed knife, had the drug PCP in his system.

By the time Van Dyke arrived, police cars had McDonald effectively surrounded in a quiet area in the early hours. But Van Dyke opened fire within seconds and most of the shots were fired after McDonald fell to the ground.

The jury was made up of one African American woman, one Asian man, three Latino women, four white women and three white men. On Friday, the families of Van Dyke and McDonald were present in the fifth-floor courtroom. Before the verdict was read, Judge Vincent Gaughan said the verdict would not be easy “for either side”. But he told those in court to control their emotions, warning: “If you do act up, I guarantee, I’m gonna arrest you.”

Van Dyke, sitting at the defence bench in the well of the court, took a long swig from a bottle of water as the verdicts were read. His expression remained fixed. The judge revoked Van Dyke’s bail and ordered a pre-sentence investigation, before adjourning the case until 31 October. Van Dyke is now in custody.

Special prosecutor Joseph McMahon had told the jury that “under very special circumstances” a police officer can legally shoot a person but Van Dyke was charged with first-degree murder because “not a single shot was necessary or justified”.

“Someone needed to arrest Laquan McDonald, not stop him with a hail of gunfire,” McMahon said in his closing argument on Thursday.

Van Dyke was the first Chicago police officer to face trial for killing someone on duty in 50 years. The defence argued that he was not a murderer but a “scared police officer, fearful for his life and others and he acted within his training”.

Defence attorney Daniel Herbert called it “unprecedented” for an officer to be charged with murder for doing his job to stop an armed offender. “You can use your common sense, ladies and gentlemen. You can determine what is a murder. This isn’t,” he told the jury in his closing argument.

Mayor Rahm Emanuel, Barack Obama’s former White House chief of staff, faced calls for his resignation amid claims he covered up the dashcam video until after he was re-elected. He was due to run for a third term, but announced last month he would step down.

Lori Lightfoot, Democratic candidate for mayor and co-chair of the city’s police reform task force, called the verdict “a significant milestone in Chicago’s history”.

In a statement, Southwood of the police union said: “This is a day I never thought I’d see in America, where 12 ordinary citizens were duped into saving the asses of self-serving politicians at the expense of a dedicated public servant.

“This sham trial and shameful verdict is a message to every law enforcement officer in America that it’s not the perpetrator in front of you that you need to worry about, it’s the political operatives stabbing you in the back. What cop would still want to be proactive fighting crime after this disgusting charade, and are law abiding citizens ready to pay the price?”