Binyamin Netanyahu has opened his campaign for the leadership of Israel's ruling Likud party with an attack on Prime Minister Ariel Sharon.
Current opinion polls put Mr Netanyahu ahead
During a tour of the West Bank settlement of Maale Adumim, the former premier said Mr Sharon was jeopardising Israel's hold on Jerusalem.
He accused the prime minister of turning Gaza into a "terrorist base" following Israel's disengagement.
Mr Netanyahu quit as finance minister earlier this month over the Gaza issue.
Israeli has closed all its settlements in the Gaza Strip and four in the West Bank.
"Sharon created a precedent that could lead to a division of Jerusalem," Mr Netanyahu said in Maale Adumim, on the outskirts of Jerusalem.
"Starting my campaign here is not coincidental because Jerusalem is in danger."
Mr Netanyahu said that if he was prime minister he would build thousands of homes in an area known as E-1, east of Jerusalem, in order to connect the city with Maale Adumim.
He also accused Mr Sharon of "giving in to international demands" on settlement building in the Jerusalem area.
Israel has been rebuked by Washington over its continued building work in the West Bank, which contravenes the US-backed roadmap plan for peace.
Palestinians argue that building in the area will cut off East Jerusalem from the rest of the West Bank and prejudice any final status negotiations on the city, which Palestinians hope to make their future capital.
Mr Netanyahu announced his candidacy for the Likud leadership on Tuesday.
Currents polls put him ahead of Ariel Sharon in the contest, in which he and his followers are looking to punish the prime minister over the Gaza disengagement, correspondents say.
Likud's central committee is due to meet on 25 September to decide a date for the start of the leadership vote, expected to be in November.