Egypt's National Salvation Front, a coalition of the country's largest liberal-democratic parties and movements, has officially rejected talks proposed by President Mohamed Morsi.
Morsi addressed Egyptians Thursday urging a national dialogue between all political forces, but the opposition said the president had not offered enough concessions.
Instead, Morsi's opponents said they would take to the capital's streets to protest the president's recently expanded powers, Saudi-owned news channel Al Arabiya reported.
Two other opposition groups said they would also boycott the talks, BBC reported citing state media.
Morsi's Nov 22 constitutional declaration expanded his executive authority by barring the courts from challenging his decisions. Egypt's highest judicial authority, the Supreme Judicial Council, earlier called Morsi's move "an unprecedented attack on the independence of the judiciary and its rulings".
Opposition groups have been rallying against the constitutional declaration and Morsi's decision to hold a nationwide referendum on the new Constitution Dec 15, which they described as too hasty.
Earlier this week, violent clashes between Morsi's supporters and opponents left five people dead and hundreds injured.