Jennifer Kerns, the spokeswoman for "Yes on 8: Protect Marriage," said proponents planned to file the lawsuit in Sacramento County Superior Court at 3 p.m. today.
Brown changed the wording on the ballot to say that the initiative would "eliminate the right of same-sex couples to marry."
The original wording stated that a section would be added to the constitution to provide that "only marriage between a man and a woman is valid or recognized" in California.
If passed, the initiative would overturn a May 15 ruling in which the California Supreme Court said that the California Constitution provides a right to gay and lesbian marriage.
"Proponents of Prop 8 feel that the ballot title and summary that Attorney General Brown assigned to Prop 8 are extremely argumentative and prejudicial," Kerns said.
"The attorney general's job is to assign ballot summaries that are fair, accurate and impartial, and in this case we believe that Attorney General Brown has assigned one that is extremely prejudicial against the measure," she said.
Kerns said the campaign is requesting that a judge set a hearing date as early as this week, and that the group would like for the original wording to be restored to the initiative.
"When we circulated this measure for signatures, and got 1.2 million of them, Attorney General Brown had titled it quite accurately," Kerns said. "But now that it's been approved, he's done a complete about-face."
The attorney general's office did not have an immediate comment on the lawsuit.
The action by proponents of the initiative came on the same day that its opponents received a financial boost and a public declaration of support from Pacific Gas and Electric Co.
At a news conference at its San Francisco headquarters this morning, the utility announced a $250,000 donation to the campaign against Proposition 8 and established itself as the founding member of a business advisory council that will work to defeat the initiative.
PG&E officials were joined by various gay and lesbian rights groups opposed to the proposition.
Nancy McFadden, senior vice president of public affairs at PG&E, brought dozens of PG&E employees on stage as she presented the check to members of the "No on 8" campaign.
"We're not only putting words, but now putting money behind the fight," McFadden said.
McFadden said that the decision to contribute financially was made in the last month, and that the money is a portion of the company's shareholder dollars that is put aside for political contributions.
PG&E also announced the founding of the Equality Business Advisory Council, an effort to involve businesses in the push to defeat Proposition 8.
"We're sending letters and making calls, and by the end of the week we should have other businesses involved," McFadden said.
Also speaking at the conference were members of Equality California and the National Center for Lesbian Rights.
"We're at this amazing moment in history where we have true equality for all LGBT people," said Geoff Kors, executive director of Equality California.
"We're here because businesses have stood with us since the beginning of this fight. Those are the partnerships that got us to this moment and that's what's going to help us defeat Prop 8," Kors said.
Kate Kendall, executive director of the National Center for Lesbian Rights, spoke after McFadden presented the check to the campaign.
"We're changing the future and PG&E understands that," Kendall said. "They now lead the way in the business world, and we know others will follow."