Sarkozy stepped into a car to head to the French capital's La Santé jail to start a his sentence, in a stunning downfall for a man who was president of France between 2007 and 2012.
A court decided to jail the conservative former leader after finding him guilty of conspiring to raise campaign funds from Libya.
Sarkozy will become the first former French leader to be jailed since Nazi collaborator Marshal Philippe Petain after World War II.
"I want to tell (French people), with the unshakable strength that is mine, that it is not a former president of the Republic who is being imprisoned this morning — it is an innocent man," he wrote in a long message on social media platform X on Tuesday.
Sarkozy's conviction caps years of legal battles over allegations his 2007 campaign took millions in cash from Libyan leader Muammar Gaddafi, who was later overthrown and killed during the Arab Spring uprisings.
While Sarkozy was found guilty of conspiring with close aides to orchestrate the scheme, he was acquitted of personally receiving or using the funds.
He has consistently denied wrongdoing and called the case politically motivated.
"I am very proud of him, proud that he is going to prison with his head held high, and absolutely convinced of his innocence," his brother Guillaume Sarkozy, who was among relatives and supporters who cheered Sarkozy on his way to jail, told BFM TV.
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