Egypt's public prosecutor on Thursday ordered a probe into the top three leaders of the opposition on suspicion of trying to incite followers to overthrow President Mohamed Morsi, a legal source said.
The prosecutor, Taalat Ibrahim Abdallah, who was appointed by Morsi late last month, signed the order against the leaders of the opposition National Salvation Front, which led protests against Morsi's drive to have a new constitution adopted.
The probe targets Mohammed ElBaradei, a Nobel peace prize laureate, Amr Moussa, former chief of the Arab League, and Hamdeen Sabbahi, the leader of the nationalist left wing. Moussa and Sabbahi were presidential candidates in June elections that Morsi won.
The National Salvation Front alleged frauds and irregularities in the December 15 and 22 split referendum on the new charter, which Morsi signed into law this week.
It accuses Morsi's Muslim Brotherhood of wanting to use the constitution to introduce creeping strict Islamic sharia law.
Abdallah called on Justice Minister Ahmed Mekki to name an investigating magistrate for the probe, which would examine suspicions of "inciting for the overthrow of the regime".
Morsi on Wednesday hailed the adoption of the new constitution with 64 percent of the votes in the referendum, though turnout was a low 33 percent.
Within two months, Egypt has to hold legislative elections to choose a parliament to succeed the one dissolved by the constitutional court in June. The opposition parties in the National Salvation Front coalition are considering competing in the elections on the same ticket.