His ex-wife and co-defendant Patricia Menard, with whom he ran a lucrative hair transplant clinic, arrived separately.
Nearby, dozens demonstrated against the impunity of the banking system, with one cigar-chomping protester in top hat and tails impersonating a banker.
Cahuzac and Menard face up to seven years in jail and two million euros ($2.2 million) in fines if found guilty of tax fraud and money laundering.
The defence opened by challenging the constitutionality of the case, arguing that Cahuzac has already settled his debts and should not be tried twice over the same matter.
"Even if the press calls someone a 'pariah' we can still follow the law, and (the double jeopardy recourse) is a question of law," said Cahuzac's lawyer Jean Veil.
Both Cahuzac and Menard have paid back taxes and penalties, their lawyers argued. The court was told she had returned more than two million euros although no figure was given for the ex-minister.
If the question is referred to the constitutional court, the trial could be delayed by several months. If not, it is scheduled to end on February 18.
The scandal was the first of a series that have tarnished the presidency of Francois Hollande, who had promised a squeaky clean government after succeeding Nicolas Sarkozy, the subject of several graft investigations, in 2012.
To begin with, Cahuzac vehemently denied the allegations, notably before parliament and to Hollande, after the Mediapart news website broke the story in December 2012.
Cahuzac, whose remit included fighting tax fraud, lodged a defamation suit against Mediapart.
But the trained surgeon resigned his post after a formal investigation was launched in March 2013.
Two weeks later, he confessed to having held an account with Swiss banking giant UBS and said he was "consumed by remorse".
The scandal prompted Hollande to order his ministers to disclose their personal wealth, a first in France, where personal finances are rarely discussed and the wealth of public officials had long been considered a private matter.
But many, including Cahuzac himself, say he has served as a scapegoat as tax havens continue to conceal riches, not just for individuals but also for corporations, notably banks.
The protesters outside the court Monday, who partially blocked a bridge over the Seine with chairs stolen from banks, said Cahuzac's case did little to change the status quo.
"They say the Cahuzac affair was an electroshock, but only small steps have been taken," said activist Thomas Coutrot, claiming: "French banks are massively present in tax havens."
Prosecutors described the Cahuzacs' swindle as "determined" and "sophisticated" as well as a "family affair" that also involved Cahuzac's mother, although she is not on trial.
Also in the dock are the divorced couple's erstwhile advisers, Swiss banker Francois Reyl and Dubai-based lawyer Philippe Houman.