"Efforts are being made to once again make an imposition on the Palestinian people in the name of peace. ... The result of all conferences held in the name of peace so far have been to the detriment of the Palestinian nation," Supreme Leader Ali Khamenei said in a speech marking the end of the Muslim holy month of Ramadan.
While releasing few details, American officials have said the meeting will be a serious attempt to create peace between Israel and the Palestinians. Israel and Palestinian President Mahmoud Abbas hope to present the contours of a final peace accord, but Palestinian and Israeli officials hold widely different expectations of what it is meant to achieve.
Palestinians are calling for a detailed preliminary agreement with a timetable for creating a Palestinian state. Israel is pressing for a vaguely worded document that would gloss over the toughest issues - borders, control over disputed Jerusalem and a solution for Palestinian refugees who lost their homes in the 1948 war that followed Israel's creation.
Iran doesn't recognize Israel and wants the establishment of a Palestinian state with Jerusalem as its capital. Khamenei, who has the final say on all state matters in Iran, has repeatedly called Israel a "cancerous tumor" that needs to be removed from the Middle East.
The head of the Iran-backed Palestinian Hamas government in the Gaza Strip, Ismail Haniyeh, also has urged Arab countries not to attend the conference.
"Palestinians have rejected this (conference). How can other governments attend this conference?" Khamenei said, referring to the Islamic militant Hamas.
There is also growing skepticism of the conference among some Arab governments, which have expressed doubts the planned gathering will tackle the main issues of the conflict with Israel.
The Bush administration has said it will invite its Iran-allied adversary Syria to the conference, but Syrian President Bashar Assad has all but ruled out his country's participation. Egypt, Jordan and Saudi Arabia have not said whether they will attend.
Hamas seized power in Gaza from Fatah security forces loyal to Abbas in June. Abbas retaliated by expelling Hamas from government and setting up a Western-backed government in control of the West Bank.