Egypt’s chief prosecutor ordered an investigation on Thursday into allegations that opposition leaders committed treason by inciting supporters to overthrow Islamist President Mohammed Morsi.

The probe by a Morsi-appointed prosecutor was launched a day after the president called for a dialogue with the opposition to heal rifts opened in the bitter fight over an Islamist-drafted constitution just approved in a referendum. The opposition decried the investigation as a throwback to Hosni Mubarak’s regime, when the law was used to smear and silence opponents.

The probe was almost certain to sour the already tense political atmosphere in the country.

The allegations were made initially in a complaint by at least two lawyers sent to the chief prosecutor earlier this month. They targeted opposition leaders Mohammed ElBaradei, a Nobel Peace laureate and former head of the U.N. nuclear agency, former Foreign Minister Amr Moussa, and Hamdeen Sabahi. Both Moussa and Sabahi were presidential candidates who competed against Morsi in the last election.