The National Salvation Front, the main opposition bloc in Egypt, announced that it would not participate in parliamentary elections scheduled to start in April, saying there must first be a law guaranteeing a free and fair vote.
"The decision of the front, unanimously, is to boycott the elections," member Sameh Ashour told reporters on Tuesday after a meeting of the coalition, which groups mainly liberal and leftists parties and movements.
The elections were called last weekend by president Mohamed Morsi. The vote will start in April and be staggered over a two-month period.
Ashour said the decision to boycott the vote was taken unanimously. The opposition has been calling on Morsi to defuse the nation's political crisis before calling for elections to prevent the country from plunging deeper into chaos.
"There can be no elections without a law that guarantees the fairness of the election process and a government that can implement such a law and be trusted by the people," Ashour said, adding that there must also be "real independence of the judiciary."
'Talk with yourself'
It wants a "neutral" government to replace the one led by prime minister Hesham Qandil, and for steps to be taken to ensure the independence of the judiciary.
Divisions between Islamist parties and their opponents have widened since Morsi won last year's presidential election.
Tensions spilt into lethal street violence late last year when the president was accused of staging a power grab, an accusation dismissed by the ruling Freedom and Justice Party as propaganda.
Seeking to convince the opposition to take part, Morsi had invited them to talks on Tuesday to address concerns about the vote. The NSF said it would not attend.
"We tell President Morsi: talk with yourself and your party," Ashour said.
The parties involved in the boycott include the Popular Current, led by Hamdeen Sabbahi, a leftist who came third in last year's presidential election, and the Dostour party, headed by former UN nuclear agency chief Mohamed ElBaradei.