Sarkozy and 13 others are accused of setting up or benefiting from a fake billing scheme which enabled the concealment of millions of euros in excess spending during the 2012 presidential campaign against Socialist rival François Hollande.
Candidates in French elections are allowed to spend a strictly limited amount on campaign expenses.
Prosecutors say accountants warned Sarkozy that his 2012 campaign was set to exceed the 22.5 million euro spending cap, but that he insisted on holding more events.
Eventually the campaign cost nearly 43 million euros, though Sarkozy says he was unaware of any scheme to conceal the 20 million euro overspend. Unlike some of the defendants, the former president is not charged with fraud, but with the lesser offence of illegal campaign financing.
If convicted, he risks up to a year in prison and a fine of 3,750 euros.
Sarkozy is 66 years old.
The trial was originally set for March but was postponed after a lawyer for a key witness was hospitalised with Covid-19. It is now set to run until 22 June.
Sarkozy did not appear in court when the trial opened in Paris on Thursday afternoon, but he has been ordered to appear for questioning in the week of 14 June.
Victim of 'a vindictive judicial system'
The case is one of several which have dogged Sarkozy since he left office.
He has denied any wrongdoing, saying he is the victim of a vindictive judicial system that widely opposed his reform efforts while in power between 2007 and 2012.
The latest case is known as the Bygmalion affair, named after the public relations firm hired to orchestrate a blitz of elaborately staged rallies when polls showed that Sarkozy's 2012 re-election was far from assured.
Bygmalion executives have acknowledged a system of fake invoices to pass the bills to Sarkozy's UMP party, since renamed Les Republicains.
The deputy manager of the campaign, Jerome Lavrilleux, made headlines in 2014 after he tearfully confessed to the scam during a French TV interview, saying: "This campaign was a runaway train that no one had the courage to stop."
Campaign officials refused to reimburse the excess spending after investigators discovered the fraud, prompting the UMP to launch a "Sarkothon" that raised 11 million euros to cover legal costs.
A popular right-wing icon
Sarkozy is also facing charges that he received millions of euros from former Libyan dictator Moamer Kadhafi to finance his 2007 election campaign.
In January, prosecutors opened a probe into alleged influence-peddling involving the former president's activities as a consultant in Russia.
Sarkozy remains a popular figure on the right, attracting long lines of fans last summer seeking autographs as he launched his latest memoir, "The Time of Storms," which topped best-seller lists for weeks.
