JERUSALEM, July 17 (Reuters) - An Israeli high court justice slammed state prosecutors on Tuesday over a plea deal that dropped a rape charge against the former president in exchange for his admission to lesser charges of sexual assault.

"Your tactical switch is incomprehensible, there is no logical explanation," Israeli media quoted Justice Ayala Procaccia as saying in a hearing on an appeal by womens' advocacy groups against the deal last month with then-President Moshe Katsav.

The court postponed a decision on the appeal, further delaying the start of Katsav's trial in a case that has brought unprecedented disgrace on an Israeli head of state and highlighted complaints of pervasive sexual harassment at work.

Procaccia questioned Attorney-General Menachem Mazuz's decision to drop a rape charge against Katsav in exchange for his resignation last month and admission to committing indecent acts against a woman employee and of sexually harassing another.

Mazuz had said months ago he had evidence to try Katsav for raping another employee, but then opted to drop the case. He said there was conflicting evidence, though he branded Katsav a "serial sex offender" in the same television interview.

"The impression I get is that prosecutors did two separate risk evaluations," based on the same evidence, Procaccia said. "There was in effect nothing objective here" about the evidence, she added. "The only change was in the risk threshold."

In an interview with Channel 2 television, his sole public appearance since the plea deal, Katsav declined to admit any guilt and portrayed himself as a victim of "an entire year of incitement and persecution".

Katsav stepped down late last month and has been succeeded by elder statesman Shimon Peres, who was sworn in on Sunday.

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