Recent polls have shown increased support for Le Pen although he is lagging behind Segolene Royal, the socialists' presidential hopeful, and Nicolas Sarkozy, the conservatives' likely candidate.
Both Royal and Sarkozy have courted voters with tough law-and-order policies in what commentators say is an effort to win over Le Pen voters.
Le Pen confident
The 78-year-old Le Pen said he had overcome his differences with Megret for the "love of our homeland".
"I think this is the start of a big popular movement, which could have a decisive impact on the presidential election," Le Pen told a news conference with Megret.
In past months, Le Pen has sought to reshape his image and broaden his appeal. A new campaign poster does not, as usual, portray Le Pen himself, but features a woman of apparent immigrant origin.
Le Pen has yet to secure the 500 signatures of support from mayors across France needed to run.
Megret said he could provide Le Pen with some 140 signatures if necessary.
"I will not allow the French to only have a choice between a vote for Sarkozy and a vote for Royal in the second round of the presidential election," Megret said.