Lacazette scored twice inside the first 30 minutes and 21-year-old midfielder Nabil Fekir added the other just after the break to make it eight league goals in 16 games.
"The team is young but we have a lot of talent," Fekir said. "We're happy but we're just starting the second half of the season. There's still work to be done."
The rise of Lyon, which won seven straight titles until Bordeaux ended its reign in the 2008-09 campaign, is even more remarkable given that it lost three of its opening four league games.
"I measure how far we've come," Fournier said. "The club sticks together in difficult moments and there's a real collective strength. Maybe we had to experience these tough times to get where we are."
A minute's silence was held before all matches this weekend in memory of the 17 people killed in Paris this week, following three days of bloodshed in a rampage by terrorists. As Lyon prepared to play at Stade Gerland, millions lined the streets in Paris, Marseille, Bordeaux, Toulouse, Nice and other cities in peaceful marches that have helped an oft-fraught nation pull together to solemnly honor the victims.
"It was a hard match to prepare given the heavy context," Fournier said. "The players were particularly touched by what was happening."
Although Lyon has scored 43 goals in 20 league games, coach Hubert Fournier tried to play down the growing expectations that Lyon can challenge for the title.
"The role of the coaching staff is to detect the slightest bit of complacency," Fournier said. "You have to be very vigilant."
Meanwhile, fifth-place Monaco is five points behind PSG " which lost 4-2 at Bastia on Saturday " after drawing 0-0 at home to sixth-place Bordeaux, and seventh-place Nantes drew 0-0 at home to Metz.
Having converted from a right-winger to a central striker, Lacazette has been one of the most prolific strikers in European football, and has netted 11 times in his past seven games.
Fekir, a gifted passer as well as a confident finisher, set up Lacazette's first goal in the 14th minute.
His quick pass found Lacazette lurking toward the right of the penalty area, and he confidently drilled a shot inside the near post.
When midfielder Corentin Tolisso's pass deflected off Serbian defender Uros Spajic and fell kindly to Lacazette, he beat goalkeeper Ali Ahamada to make it 2-0 in the 27th.
Fekir made it 3-0 in the 48th minute when he latched onto a pass from midfielder Jordan Ferri and scored with a low shot from the right of the penalty area.