Lula is seeking a fourth non-consecutive term in office but is facing stiff competition from his right-wing challenger, the son of jailed ex-leader Jair Bolsonaro.
Flavio Bolsonaro received 47 percent of the vote over the incumbent's 45 percent in the first round earlier this month, and polls show a technical tie ahead of Sunday's runoff.
The petition was initially expected to be submitted Tuesday to Brazil's top electoral court concerning alleged "algorithmic manipulation" by Meta aimed at harming Lula's chances of winning, the campaign source said.
But Lula's campaign coordinator, Senator Camilo Santana, later told reporters the filing would occur Wednesday.
The information was first revealed Monday by Brazilian newspaper Folha de S. Paulo.
Meta and lawyers from both campaigns met at the Superior Electoral Court (TSE) on Tuesday, a court source said.
The court on Monday gave Meta 24 hours to prove it was complying with an order to treat both campaigns equally, threatening a daily fine of around $200,000 if it didn't.
The order came after Lula's campaign alleged that Meta, which owns Facebook, WhatsApp and Instagram, was suppressing his online content to boost Bolsonaro.
"We have completed the implementation of the measures to comply with the Superior Electoral Court's decision," the company stated on Tuesday.
- Lula seeks fightback -
Lula's campaign also alleged inconsistencies in WhatsApp's artificial intelligence tool, saying Lula-related election content was limited while Bolsonaro content was untouched.
Meta had denied taking any actions to favor one side in the election, but previously said that some of Lula's campaign ads had been mistakenly deactivated due to technical errors.
Among measures the tech giant adopted, Meta said that now only candidates and parties registered with the electoral court can place ads on its networks.
The company also said it removed 720 posts containing AI-generated content that lacked proper labeling. Meta's London-based oversight board also said it had identified recurring failures with content about Brazil's presidential elections and the US congressional elections.
Dozens of Lula supporters protested outside the company's Sao Paulo offices on Monday, bearing placards and chanting slogans like "Meta out of Brazil."
Lula, 80, has also accused the American government of meddling in the elections in favor of Flavio Bolsonaro.
US President Donald Trump has not yet explicitly endorsed Bolsonaro, 45, but called his surprise first-round finish a "very, very big victory."
The runoff victor will either preserve the vast country of 214 million people as a left-wing stronghold in Latin America, or usher it into the ranks of right-wing administrations loyal to Trump.
The shock result of the first round appears to have invigorated the left, with Lula's camp staging massive rallies ever since and seeking to up its social media game.
Flavio's father Jair Bolsonaro is serving a 27-year prison sentence under house arrest for attempting a coup following his electoral defeat to Lula in 2022.
Flavio Bolsonaro has sworn to liberate his father if elected.
Lula or Bolsonaro's son? Brazil makes a final choice