The reaction of the two leading candidates could not have been more different. Ms. Le Pen blamed government “laxity” and “naïveté” toward terrorists, while Emmanuel Macron, an independent centrist, said the terrorist threat “will be a fact of daily life in the coming years.”

It is impossible to know how the attack will affect the election because so many people remain undecided, and may in the end submit blank ballots — a popular form of protest in France. Some 30 percent of registered voters may not show up at all — compared with 20 percent in the first round of voting in 2012 — making it likely that abstention will be the most popular choice in the early balloting.

The latest polls show Ms. Le Pen and Mr. Macron in a virtual tie, though Mr. Macron edged ahead on Friday with 24 percent to Ms. Le Pen’s 22.5. Mr. Macron, of the new centrist party En Marche! (or Onward!), is staunchly pro-European Union and promises to be both business-friendly and a protector of France’s generous social programs. But he suffers from having been minister of finance under President François Hollande, who is so unpopular that he isn’t running for re-election. The candidate of Mr. Hollande’s Socialist Party, Benoît Hamon, is trailing badly, with a projected 7 percent of the vote.

Close behind Mr. Macron and Ms. Le Pen are the center-right Républicains candidate François Fillon and the far-left populist Jean-Luc Mélenchon. They are also virtually tied, with Mr. Fillon at 19.5 percent and Mr. Mélenchon at 18.5. Mr. Fillon, who was the front-runner after an upset in his party’s primary, lost support after it was revealed that he had paid his wife and children around $1 million in public money that they may not have earned. Mr. Mélenchon had an astonishing last-minute surge after performing well in televised debates, and in a show of how modern, even futuristic, he is, his campaign has used holographic appearances to simultaneously reach supporters in multiple cities.

While people interviewed at the market were unanimous in their aversion to Ms. Le Pen, they were split on whom they were for. That could be a sign of how Pantin, the “Brooklyn of Paris,” has changed with the arrival of arty types and young families who can’t afford Paris proper. In addition, some were adamant that if their candidate didn’t make it to the second round, they would not be able to choose another. One man said, “If Fillon isn’t on the final ballot, my decision is simple: I’m going fishing.”