Katsav has denied the allegations and insists he is innocent of assaulting the four women who have accused him, but he has faced widespread calls to resign after Menachem Mazuz, the attorney general, announced he would be charged.
Zehava Galon, a member of the Meretz Party, said Katsav should have immediately resigned on Tuesday when Mazuz confirmed his intention to charge the president.
Galon said she would start collecting the 20 politicians' signatures necessary to convene an impeachment debate in parliament, without waiting for Katsav's statement.
She told Israeli Radio: "We are starting the process of signing up members of parliament to bring about the impeachment of the president."
"After examining all the available evidence, the attorney general has reached the conclusion that there is sufficient alleged evidence to file an indictment against the president," Israel's justice ministry said on Tuesday.
A final decision on the indictment will be made only after a hearing, where Katsav will be allowed to present his case.
Israeli media, politicians and the public were nearly unanimous in their opinion that Katsav should go.
"Someone should explain to the honoured president that he should go home," wrote the second-largest daily, Maariv.
"Release his grasp on the trappings of power, pack his bags and take his leave of the president's residence in Jerusalem as quickly as possible."
Yossi Beilin, leader of the Meretz Party, said: "He should resign as he has no right to leave us in this situation.
"As a society, we have the right to tell him that he is no longer our president and that his portrait can no longer be hung in schools."
Guidon Saar, the leader of the Likud opposition party, said: "The president must resign. Period. And I hope that he will take such a decision within the upcoming hours. If not, the ball will be in Knesset's court, which should fire him."
A poll published in Yediot showed that 79 per cent of Israelis had little or no confidence in the Iranian-born Katsav, who was elected by MPs in the Knesset in 2000.
Katsav is due to be indicted with raping an employee at the tourism ministry where he held the top post in the late 1990s. He will face separate charges of sexually harassing three other employees at the presidential residence.
He also faces charges of offering "dozens of silver cups" as gifts while serving as president and of obstructing the investigations into his activities, the ministry said.
He faces the prospect of becoming Israel's second consecutive president forced out by scandal.
The late Ezer Weizman, Katsav's predecessor, was forced to resign in 2000 after allegations he received around $450,000 as "gifts" from French millionaire Edouard Saroussi in the 1980s, when Weizman was a member of parliament and minister.
Source: Al Jazeera and agencies