ISTANBUL (AFP) - Turkish President Recep Tayyip Erdogan on Thursday urged dozens of Muslim leaders gathered for a summit in Istanbul to end sectarian divisions in the Islamic world and join forces to fight terror.

Turkey is seeking to showcase its influence among the world's estimated 1.7 billion Muslims, particularly in lands once controlled by the Ottoman Empire, at the two-day summit of the Organisation of Islamic Cooperation (OIC) which it will chair for the next two years.

But the meeting bringing together over 30 heads of state and government has been shadowed by sectarian-tinged conflicts in Syria and Yemen that have pitted Shia Muslims - led by regional power Iran - against Sunni Muslim powers like Saudi Arabia.

Key guests at the summit included Saudi King Salman and Iranian President Hassan Rouhani, in a rare public encounter although there were reports the two men had exchanged words.

"I believe the greatest challenge we need to surmount is sectarianism. My religion is not that of Sunnis, of Shiites. My religion is Islam," Erdogan said in his opening speech.

"We should be uniting. Out of the conflicts, the tyranny, only the Muslims suffer," he said, adding the summit meeting could be a "turning point" for the whole Islamic world.

Erdogan lashed out at Islamic State in Iraq and Syria (ISIS) extremists who seized swathes of Syria and Boko Haram Islamist extremists in Nigeria as two "terrorist organisations that are serving the same evil purpose". He said that the OIC had accepted a Turkish proposal to set up a multinational police coordination centre for Islamic states to fight militants, to be based in Istanbul.

"We need to establish an organisation to further strengthen cooperation in the fight against terror," he said.

A security lockdown has been thrown around the summit venue in Istanbul, the former capital of the Ottoman Empire from where the Sultans for centuries ruled Muslims from the Balkans to Arabia.

Some 5,000 extra police have been deployed in Istanbul to ensure the event passes smoothly after two deadly suicide attacks blamed on extremists in Istanbul this year alone.

But Turkey's own policies in the Middle East have been controversial, with several Muslim states objecting to the Islamic-rooted government's backing of rebels in Syria.

While the summit marks one of the most significant gatherings of heads of state for years in Istanbul, some high profile leaders like Jordan's King Abdullah and Egyptian President Abdel Fattah al-Sisi are notable by their absence.

Turkey's relations with Cairo have still not recovered from the 2013 ousting of Islamist president Mohamed Morsi, a close ally of Ankara, while ties with Amman are being tested by differences over Syria.

Turkey took over the chairmanship of the OIC from Egypt, whose Foreign Minister Sameh Shoukry read a brief message from Sisi emphasising Cairo's commitment to the group.

Shoukry conspicuously made no reference to the Turkish president in his speech but Erdogan also pointedly thanked Cairo for its efforts.

The run-up to the summit saw a landmark visit by King Salman to the Turkish capital Ankara which highlighted the dramatic improvement in ties between Turkey and Saudi since he came to the throne in 2015.

Overseen by Erdogan and Salman, the two countries' foreign ministers on Thursday signed a memorandum on creating a Saudi-Turkish Coordination Council to further deepen bilateral relations.

Addressing the summit, Salman said "we are obliged today, more than ever, to fight terrorism" and appeared to lash out at Teheran, without naming the Islamic Republic.

He denounced "flagrant interference in the affairs of several Islamic countries... instigating sedition and divisions, inciting sectarianism and using armed militia to undermine our security", according to the official SPA agency.

Saudi Arabia and Turkey both believe the ousting of President Bashar al-Assad is the key to solving the Syrian conflict and back rebel groups fighting his regime.

Analysts have warned however that Turkey needs to tread carefully in its alliance with Saudi Arabia, which is also overwhelmingly Sunni Muslim, so it is not seen as a sectarian union aimed at Shiite Iran.

In a sign of Ankara's desire to maintain a delicate balance, Rouhani is due to begin a bilateral visit to Turkey after the summit.