The environmental group said it is seeking a restraining order from a federal court in Angra dos Reis, a coastal resort south of Rio de Janeiro and the site of Brazil's only two nuclear power plants.
Another action was filed in the nation's capital, Brasilia, Greenpeace press spokeswoman Gabriela Michelotti said.
Public prosecutor Andre de Vasconcelos Dias in Angra dos Reis was traveling and not available for comment, his office said.
The existing nuclear plants, Angra 1 and Angra 2, have a combined installed capacity of about 2,000 megawatts. Angra 3 would raise capacity to 3,300 megawatts at a cost of about US$3.6 billion, according to the Mines and Energy Ministry.
In August, the government moved to restart work on Angra 3, which has been stalled since the 1980s by lack of funds. The government said this year it could build an additional four to eight nuclear power plants after Angra 3.
Greenpeace issued a statement that it's court filings name the federal government, the Eletronuclear power company, the Ibama environmental protection agency and the Rio de Janeiro State Environmental Engineering Federation, or Feema, claiming that the construction of Angra 3 is "illegal and unconstitutional."
Greenpeace said the plant lacks the express authorization of President Luiz Inacio Lula da Silva and the approval of Congress.
The Associated Press telephoned Eletronuclear and Ibama but was told that no one was available to comment. A call to Feema went unanswered.
Brazil needs energy to fuel its expanding economy. In July, Ibama gave preliminary approval to a US$10 billion to US$14.7 billion project to build two dams in the Amazon that would generate 6,450 megawatts, or 8 percent of current electricity demand. Critics of nuclear energy say Brazil should use more solar, wind and biomass energy.