The elections were called last weekend by Islamist President Mohammed 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. It wants a "neutral" government to replace the one led by Islamist Hesham Kandil, and for steps to be taken to ensure the independence of the judiciary.