A major police union in Italy has provoked outrage after giving a five-minute standing ovation to three officers convicted of the manslaughter of an 18-year-old.

The mother of Federico Aldrovandi, who died in the northern town of Ferrara in September 2005 on his way back from a night out, said she had felt both "fear and disgust" upon hearing of the warm reception given to the officers by the Autonomous Union of Police (SAP) on Tuesday.

The three men – Paolo Forlani, Luca Pollastri and Enzo Pontani – were definitively convicted of manslaughter by Italy's supreme court in June 2012 and sentenced to three years and six months in jail for their role in Aldrovandi's violent death. Another officer, Monica Segatto, was also convicted but was not present at the conference of SAP, the second-largest police union.

Due to a law aimed at easing prison overcrowding, the sentences were commuted to six months. None of the officers has been fired and they returned to work, albeit in office-based roles, in February – a development that prompted thousands to protest in Ferrara under the slogan Via la Divisa (Get rid of the uniform).

The standing ovation, however, illustrated that, for many of their colleagues in the police force, the officers are considered victims of a miscarriage of justice. Gianni Tonelli, general secretary of SAP, told La Repubblica: "What we are doing we are doing in the conviction that they are innocent. Otherwise we would not say anything, either as a union or as individuals."

But his words were furiously condemned by critics including Patrizia Moretti, Aldrovandi's mother. "It is awful to know that those who should be protecting legality are in fact applauding criminals convicted of manslaughter at every stage of appeal," she was quoted as saying. "The Italian justice system has deemed them [the officers] the authors of bloody violence. [SAP's] duty should not be to applaud but to defend justice and people's lives."

This is not the first time Moretti has been confronted with police anger over the convictions. In March 2013, a smaller police union held a sit-in in solidarity with the officers beneath Ferrara's city council building, where she is an employee. She came down brandishing a large picture of her dead son, his head in a pool of blood. The union later said it was not aware that she worked there.

On Wednesday, Moretti was due to meet with Alessandro Pansa, chief of Italy's state police, who distanced himself from SAP's applause, branding it "unreasonable and irresponsible". The ovation was also condemned as "shameful" by centre-left prime minister Matteo Renzi, and by centre-right interior minister Angelino Alfano as "very serious and unacceptable". Luigi Manconi, a senator for the centre-left Democratic party, blasted the police union for pursuing a "message of omertà".

Moretti told journalists that the fact the officers were able to continue in the police force and be applauded as wronged victims by their colleagues illustrated the urgent need for institutions including the police force to change their ways.

"The conviction was already done, signed off," she told journalists. "What has to change is the behaviour of institutions after a conviction for manslaughter. For the moment, it's nothing."