Turkish President Recep Tayyip Erdogan on Friday hosts for the second time in half a year a summit of the main pan-Islamic bloc to show solidarity with the Palestinians and condemn Israel after the killing of dozens of Gaza protesters.
Erdogan has reacted with unbridled fury to the killing by Israeli forces on Monday of 60 Palestinians on the Gaza border, accusing Israel of "genocide" and being run as an "apartheid state".
He has also called a mass demonstration in Istanbul expected to rally hundreds of thousands from 1300 GMT ahead of the summit's start at 1600 GMT.
Erdogan has already staged an extraordinary meeting of the Organisation of Islamic Cooperation (OIC) in December last year to denounce US President Donald Trump's decision to recognise Jerusalem as the capital of Israel.
"We must give the toughest response ... to the crime against humanity committed by Israel," Foreign Minister Mevlut Cavusoglu told a meeting of OIC foreign ministers ahead of the summit.
Speaking in Geneva, UN human rights chief Zeid Ra'ad Al Hussein slammed Israel's reaction to the Gaza protests as "wholly disproportionate", backing calls for an international investigation.
'OIC disunity upsetting'
However, as in 2017, disputes between the OIC's key players -- notably between Sunni kingpin Saudi Arabia and Shiite Iran -- may prevent the adoption of any measures going beyond harsh rhetoric.
Riyadh -- which appears to have softened its stance on Israel as the influence of powerful Crown Prince Mohammed bin Salman has grown -- and its allies fear alienating the United States with tough measures against Tel Aviv.
Saudi's chief foreign policy preoccupation, shared with Israel, is ensuring US backing to contain Iran which both Riyadh and the Jewish state see as the main threat to regional peace.
Egypt and Saudi Arabia distrust Turkey's support for Islamist movements like the Muslim Brotherhood and Hamas, complicating any effort to take concrete measures against Israel.
Cavusoglu also said some OIC member states' failure to show enough support for the Palestinian cause "upsets us".
'Dragged into chaos'
After declaring his intention to hold the event only on Monday, Erdogan has managed to build up an impressive guest list at short notice.
Jordanian King Abdullah II will be present although the Palestinians will be represented by prime minister Rami Hamdallah and not president Mahmud Abbas who this week had surgery on his ear.
From the Gulf, Kuwait's Emir Sheikh Sabah al-Ahmad Al-Sabah is expected as is Emir Sheikh Tamim bin Hamad Al-Thani of Qatar, Turkey's main regional ally.
Egyptian Foreign Minister Sameh Shoukry and Saudi counterpart Adel al-Jubeir will be at the Istanbul summit. The Saudi level of representation is higher than at the November meeting.
Iranian President Hassan Rouhani will attend, state media said, and overcoming the enmity between Tehran and Riyadh will be crucial for the Turkish hosts.
As in the November 2017 meeting, a controversial guest will be Sudanese President Omar al-Bashir, who is wanted on charges of genocide and war crimes by the International Criminal Court (ICC).
Erdogan has long craved a role as a Muslim leader within the entire Islamic world, rarely holding back with tirades against Israel even though Ankara has diplomatic relations with the Jewish state.
"If the silence on Israel's tyranny continues, the world will rapidly be dragged into a chaos where banditry prevails," Erdogan said Wednesday.
'Curse the oppression'
Tensions with Israel and hosting such a meeting also does Erdogan no harm with his core supporters as Turkey heads to presidential and parliamentary polls on June 24.
In a diplomatic crisis threatening a 2016 deal that allowed the resumption of full ties, Turkey has ordered the Israeli ambassador to leave for an unspecified period of time over the killings.
Turkey had already withdrawn its Tel Aviv ambassador for consultations while Israel ordered the Turkish consul in Jerusalem to leave, also for an unspecified period of time.
The rally expected just before the summit is set to take place at the Yenikapi meeting area favoured by Erdogan for election rallies and which has capacity for a million people.
Erdogan is himself expected to address the rally organised under the slogan "curse the oppression, support Jerusalem".