The 38-year-old Macron, once a protege of President Francois Hollande, staged what his supporters called a "tour de force" at the rally, brushing off criticism by his erstwhile Socialist allies for running as an independent in the 2017 election.
Macron has refused pleas - most recently by former prime minister Manuel Valls who is seeking the Socialist party ticket - to join in the Left's attempts to decide on a single candidate for next spring's election.
He signaled on Saturday he would not change his mind.
The former investment banker pledged to cut taxes on workers and employers, while raising taxes on income from investments and wealthy pensioners.
"I ask them, in all transparency, for this little effort of a few euros per month because the country's workers need it ... I am the workers' candidate," he declared to cheers.
The large turnout on a cold evening eclipsed last weekend's modest gathering by the Socialists when party grandees struggled to reenergize the Left's faithful at a convention that drew only 2,500 supporters.
Polls show there is little chance of any left-wing candidate reaching the election run-off next May and, barring an upset, the stage seems set for a head-to-head between conservative Francois Fillon and far-right leader Marine Le Pen.
Fillon, a center-right former prime minister who has pledged to cut deep into the public sector, would easily beat the anti-immigrant and anti-euro Le Pen, taking two thirds of the vote if the polls are correct.