Italy's far-right chief Matteo Salvini stands trial Saturday for allegedly illegally detaining migrants at sea, in a case that could see him jailed for 15 years or serve him a political win.

Prosecutors in the Sicilian city of Catania accuse Salvini of abusing his powers as then-interior minister to block 116 migrants from disembarking from the Italian Gregoretti coastguard boat last year under his so-called "closed ports" policy.

"I'll plead guilty of having defended Italy and Italians," said Salvini, who has called for his supporters to descend on Catania for a rally while he is in the dock.

"I did it... because protecting my country's dignity is my duty as a senator, as a minister, and above all as an Italian," he said on Twitter on Wednesday, next to a picture of him grinning and a countdown clock to Saturday's preliminary hearing.

Fellow far-right leader Giorgia Meloni, head of the Brothers of Italy party, has promised to attend the rally, while ex-prime minister Silvio Berlusconi, who has faced several trials himself, will send a delegation from his centre-right Forza Italia party.

Salvini, head of the far-right League, has reportedly printed up T-Shirts for the event.

The migrants, who hailed largely from Sudan, as well as central and western Africa, were rescued in the Mediterranean in two separate operations on June 25 last year after five days at sea. There were 15 unaccompanied minors among them.

- 'Treated worse than animals' -

They were transferred to the Gregoretti on July 26, then held on the overcrowded patrol vessel under a fierce summer sun -- despite a scabies outbreak and a suspected case of tuberculosis.

"Not even animals are treated that badly," Aishat Saha from Nigeria was quoted as saying by the Repubblica daily.

Saha, who was nine months pregnant at the time, was on board with her husband and their two children.

They were evacuated as a medical emergency and baby John Egidio was born just 72 hours later in Catania.

The family is a civil party in the trial and is expected in court Saturday, the daily said.

The 15 minors were eventually allowed off on July 29 following pressure from Catania's juvenile court.

The remaining migrants disembarked July 31 after Salvini, 47, said a deal had been brokered with EU countries to take them.

His defence team insists the decision to hold them was not Salvini's alone, but reached collectively within the government.

- 'Political battle' -

It will be up to preliminary hearing judge Nunzio Sarpietro to decide whether the case is strong enough to proceed with the trial.

He may not arrive at a decision Saturday, but request further preliminary hearings, Italian media said.

Analysts said the legal trouble was unlikely to hurt Salvini's popularity, but could on the contrary work in his favour.

"Those who chose to support Salvini (in the past) did so despite shady businesses," said political commentator Stefano Folli, pointing in particular to a scandal surrounding an alleged attempt to get money from Russia in a secret oil deal.

"Judicial investigations do not bother his supporters very much," he said.

Instead, the trial was "seen as an opportunity for a political battle".

Salvini's fierce "Italians First" stance saw his popularity shoot up as interior minister, though his polling numbers slid significantly during the coronavirus lockdown, which utterly overshadowed the migrant question.

The trial now looks set to pit the centre-left government, which has promised to water down his harsh security decree, against a right determined to take up the megaphone once more on one of Italy's most incendiary issues.