His statement came as US Secretary of State Hillary Clinton met Palestinian president Mahmud Abbas and premier Salam Fayyad in the Israeli-occupied West Bank on her first visit to the region since taking office.

She had met Israeli leaders in Jerusalem on Tueaday.

"The only way to save Palestine is resistance," Khamenei said in his address to open the global summit Tehran organised in aid of Gaza and the Palestinians.

"Support and help to Palestinians is a mandatory duty of all Muslims. I now tell all Muslim brothers and sisters to join forces and break the immunity of the Zionist criminals," Khamenei said.

He also called for putting Israeli political and military leaders on trial for the December-January assault on Gaza in which 1,300 Palestinians, including hundreds of children, and 13 Israelis were killed.

"This is the first step that should be taken," Khamenei said.

Iran does not recognise Israel, and Khamenei has repeatedly rejected a two-state solution to solve the Israel-Palestinian issue. Clinton reiterated support for the two-state answer this week.

Khamenei said any negotiations to solve the issue were fruitless, adding that the United States and Britain committed the "crime of creation and supporting this cancerous tumour (Israel)."

"Even the new president of the United States who came to power with the motto of changing the (George W.) Bush administration's policies talks about unconditional commitment to secure Israel. This is defending terrorism by a government."

Obama has vowed actively to pursue the Mideast peace process and has appointed a special envoy, veteran diplomat George Mitchell, to coax both sides back to the negotiating table.

Khamenei also came down hard on the United Nations, saying it covered the "illegitimate acts of bullying powers and saving Palestine will not be achieved by begging to the United Nations."

Iran's hardline President Mahmoud Ahmadinejad too called for setting up a "global anti-Zionist front."

"On the other hand we must seriously pursue punishing the Zionist criminals, so that by saving Palestine and the establishment of a popularly chosen government, the world gets rid of racism, lies and occupation forever," he told the conference.

Musa Abu Marzuk, a leader of the Islamist Hamas movement that controls Gaza and is backed by Tehran, told delegates the group "wanted national unity talks to succeed ... but a unity which satisfies the Palestinian people and not a pact which compromises their rights."

"We welcome plans to reconstruct Gaza, but if there is a condition which is against Palestinian people's principles, we will not accept it," he said.

Hamas colleague Abu Zahar acknowledged that Iran and other countries have been offering aid to Palestinians, including "paying salaries of our employees since we came to power in 2006."

Prominent Iranian lawmaker Khazem Jalili said the conference will suggest setting up a common fund of participating nations to channel the donations.

But "unfortunately, the route for delivering the aid is not open," he said referring to the Gaza crossings that have been blocked by Israel.

The summit, attended by officials from such neighbours as Kuwait and Bahrain, as well as countires such as South Africa and Nigeria, comes two days after global donors meeting in Egypt pledged to donate around 4.5 billion dollars to rebuild Gaza.

On Tuesday, Iran criticised the Egypt conference.

"Those who participated in that conference and staged that puppet show approved the Zionist regime's atrocities in Gaza," Ali Akbar Mohtashamipour, secretary general of the Iran conference, said, according to Fars agency.