Massimo Adriatici, a League councillor for security in the municipality of Voghera, was placed under house arrest on Wednesday after allegedly shooting dead 39-year-old Youns El Boussetaoui on Tuesday, following an altercation in a bar.
The victim was unarmed during the confrontation, police said.
Adriatici has suspended himself from his post and will remain under house arrest pending an investigation into the incident.
A former police commander, he had reportedly called the authorities when he believed the victim was disturbing the customers at a bar and pushed him, which led to the fight.
The politician, who was carrying a pistol, claims that a shot went off as he fell to the ground, according to Italian news agency ANSA.
The leader of the League party, Matteo Salvini, has defended the councillor saying he acted in “self-defence”.
“He is a professor of criminal law, former police officer and criminal lawyer, known and esteemed in the city of Pavia,” he said in a video posted on social media.
“He accidentally retaliated with a shot that unfortunately killed a foreign national.”
Salvini, a former interior minister whose party has adopted a hardline anti-immigrant stance, ignored calls from socialist politicians in Italy to condemn the incident and has claimed that the victim had a criminal record.
“Unfortunately, the foreign citizen was known in the city and to the police for his violence, aggression and even obscene acts in public places,” he said.
Political opponents criticised Salvini for jumping to conclusions before the police had completed their investigation.
“What happened in Voghera is a huge event, which should make everyone, including the League, reflect,” Vinicio Peluffo, head of the centre-left Democratic Party in the northern Lombardy region, said on Facebook.
“It will be up to the judicial sytem to reconstruct what actually happened, but it is necessary for everyone, without exception, to condemn the idea of do-it-yourself justice.
“Public order and ensuring compliance with the law are the responsibility of the police, certainly not of a councillor who, however you look at it, was armed in a public square.
“If the League believes that carrying a gun means safety, we are really worried, because this habit only brings bloodshed, not peace and order.”
Meanwhile, the anti-establishment Five-Star Movement (M5S) said it is “unacceptable that an unarmed man can lose his life due to a shooting in a public square, as if we were in the Wild West”.
Like other western European Union countries, Italy has tough laws regulating the ownership and use of firearms.
