Zahava Gal-On, a politician and women's rights campaigner, accused Mazuz of moral cowardice.
Gal-On said: "Victims of sex crimes will believe they do not have any shield."
Israel's high-level officials, including the prime minister, have been increasingly implicated in scandals.
Thursday's unexpected deal, which included three lesser charges, brought the five-month case to an abrupt end.
Accusations against Katsav first surfaced a year ago, and eventually spread to include four women who formerly worked with him in the president's office and earlier when he was tourism minister.
In January, Mazuz announced he planned to indict Katsav on charges of rape and lesser counts of sexual misconduct. But he allowed Katsav's attorneys to appear before him at a hearing in May to plead their case, as is customary practice with senior officials.
At that two-part hearing, new evidence was presented, he told the news conference, but did not give further details.
The attorney-general said many of the allegations expired under Israel's statue of limitations.
He did not say when Katsav would step aside, and the president's office had no details.
Under the deal, Katsav - who had insisted he was innocent of wrongdoing and the victim of a slur campaign - will plead guilty to sexual harassment, indecent acts and harassing a witness, Mazuz said.
He will pay damages to complainants, but receive a suspended sentence.
Katsav requested the plea bargain, which was finalised shortly before the news conference began, Mazuz said.
Katsav stepped aside in January to fight the allegations against him, but did not quit. Dalia Itzik, speaker of the Knesset, took over on an acting basis.
Shimon Peres, a former prime minister, who was elected earlier in the month to succeed Katsav, is to take over the job on July 15.
Kineret Barashi, attorney for one of the complainants, claims Katsav received special treatment simply because he was president.