Silvio Berlusconi's supporters have mounted a provocative protest against his conviction for paying an underage prostitute for sex and abusing his office to cover it up, amid concerns the verdict could destabilise the fragile coalition government.

As the centre-right leader prepared to meet Italy's prime minister, Enrico Letta, a number of his angry allies descended on a central square in Rome on Tuesday to hold a protest under the banner of siamo tutti puttane, which translates as "we are all whores". In advance of the demonstration, the organiser Giuliano Ferrara, editor of right-wing newspaper Il Foglio, filmed a video of himself applying lipstick.

"Let's protest against penal injustice, against hateful and hypocritical moralism, against political, media and judicial abuse," his newspaper's website exhorted.

The event ended to the strains of Ruby Tuesday, the Rolling Stones song, in honour of Karima el-Mahroug, the young Moroccan woman at the heart of the case whose stage name was Ruby Rubacuori (Ruby Heartstealer).

Francesca Pascale, Berlusconi's 27-year-old girlfriend, made an appearance at the protest, telling reporters the sentence was "disgraceful, indecent" and she felt ashamed of her country's "sick justice system".

The sentencing of the ex-prime minister to seven years in prison and a lifetime ban on holding public office has been greeted with outrage by members of the centre-right Freedom People party, who back their 76-year-old leader in his claim that he is the victim of persecution by leftwing magistrates.

The sentence is suspended pending appeal, and Berlusconi is highly unlikely ever to go to jail.

Silvio Berlusconi was sentenced to seven years in prison for paying for underage sex with the former teenage nightclub dancer Karima el-Mahroug. Photograph: Reuters

Observers say Letta's government, however, may feel its impact quickly, with the PdL portion of the coalition thought likely to make increasing demands on policies such as tax and judicial reform.

On Tuesday Italy's president, Giorgio Napolitano, chided bickering politicians and urged them to commit to continuity in government. "It hasn't been two months since the formation of a government and already the daily talk is of next, imminent or fatal government crisis," he said.

The political ramifications of the verdict look all but certain to rumble on. The Left Ecology Freedom party (SEL) called on Tuesday for the deputy foreign minister, Bruno Archi,, to resign following the inclusion of his name on a list of more than 30 defence witnesses the Milan judges said they thought should be investigated for suspected perjury during Berlusconi's trial.

The names include several political figures close to Berlusconi, a police officer and a host of models, starlets and showgirls who testified in the trial that the then prime minister's parties at Villa San Martino were good, clean fun.

It emerged during the trial that Berlusconi was making regular payments to several of the women, which his lawyers said were simply gifts to make up for the damaging impact of the trial on their careers.

If prosecutors decide there is evidence to suggest the judges' suspicions are correct, yet another chapter in the "bunga bunga" saga could be opened. No decision is expected from them before September.