Mikhael Perez, 48, one of the first people to cast his vote at a polling station in central Paris, said: "Many people seem less interested in the parliamentary elections because they think Sarkozy will win a large majority anyway.
"But I think it's important to vote," he said.
Smaller groups such as the Communists or Greens are expected to win just a handful of seats, while the far-right National Front party is not expected to pick up any seats at all.
About one million citizens in France's overseas territories and French residents in the Americas voted on Saturday. Voting began in mainland France at 8am (0600 GMT) on Sunday and first results are expected shortly after polls close at 8pm.
In his first weeks in office, Sarkozy has sought to build the image of an energetic president, promising to push through some new measures in a special summer session of parliament.
His popularity has surged since he beat Segolene Royal, the Socialist candidate, in the May 6 presidential election.
French media have talked about a "blue wave" of gains for the conservative party, washing away rivals.
It will be the third time French voters have cast their ballots in less than two months.
The Socialists have urged their supporters not to give way to election fatigue and disappointment, but to turn out in large numbers to prevent an even larger majority for Sarkozy.