Some 2,000 supporters gathered outside the Dob prison, near the capital Ljubljana, as Jansa went inside yesterday. Some carried banners comparing him to South Africa's late President Nelson Mandela.
Jansa, addressing the crowd, urged them to vote for his Slovenian Democratic Party in July 13 elections.
"On that day, we must paint Slovenia in the colours of freedom and justice," declared Jansa, who served as prime minister twice following Slovenia's 1991 independence from Yugoslavia.
Jansa and two others were convicted last year of accepting about USD 2.7 million in bribes while in office to help the Finnish firm Patria win a 2006 contract for 135 armoured personnel carriers worth USD 377 million.
He again has denied he is guilty of any offence.