The Frenchman was part of an early attack and then left his fellow escapees to win the race, formerly the Giro del Lazio, by 37 seconds from 2005 winner Filippo Pozzato with Grega Bole (Vacansoleil-DCM) in third.
On the cobblestones of central Rome, Pozzato led the group home to win the bunch sprint, and celebrated, believing he had won the race.
A five-man group featuring Kadri was established after the departure from Via dei Fori Imperial and built up an 8:45 lead as they went through the lower slopes of the Colli Albani and across the Agro Pontino.
The lead had been reduced to inside two minutes inside the final 40km when Kadri and Manuel Antonio Cardoso went clear on a challenging, hilly section.
The 26-year-old from Bordeaux then dropped the Caja Rural rider and ride alone to the finish line.
Vincenzo Nibali (Astana), Giampaolo Caruso (Katusha), Francesco Reda and Miguel Angel Rubiano (Androni) and Mauro Santambrogio (Vini Fantini-Selle Italia) had jumped out of the peloton in pursuit but they failed to bridge the gap and Kadri won against the magnificent backdrop of the Coliseum. He said: "The race wasn’t even on my programme until Friday, when my team-mate Steve Chainel fell ill. I could have gone home after Strade Bianche, but the team asked me if I wanted to ride and I said, ‘Yes.’
“I have to get into breakaways in order to win. On the Salita dei Cappuccini, I didn’t intend to attack alone. There were still 37km to go. But, on the spur of the moment, I went alone. The Caja Rural rider [Cardoso] stayed with me for a while, then I dropped him on the pavé.
“I believed in my attack. I said to myself, ‘Give it 100%. If you blow up, too bad.’ I knew there was a group of chasers, but I thought it was the remains of the breakaway, not Nibali. The final profile helped me hold off the peloton.”