Leaders of 57 Muslim countries have ended their summit with a warning that the Islamic world is in crisis because of the threat posed by terrorism.
OIC gatherings are generally platforms for state rhetoric
The Organisation of the Islamic Conference statement urges decisive action to fight "deviant ideas".
The meeting in the holy Muslim city of Mecca in Saudi Arabia called for changes in national laws to criminalise financing and incitement of terrorism.
It also called for new school curricula to purge extremist ideas.
The declaration also said that fatwas, or Islamic religious edicts, must only be issued by those who are authorised to do so.
The BBC's Middle East and Islamic affairs analyst, Roger Hardy, says Muslims attending the Mecca summit heard some heady rhetoric.
The event was hailed as a turning-point, a moment of Muslim renaissance. Using rather more mundane language, some called it a summit of moderation and modernisation.
The Mecca Declaration, read out at the end of the summit's final session, warned of the dangers of Islamic extremism.
"The Islamic nation is in a crisis. This crisis does not reflect on the present alone, but also on its future and the future of humanity at large," it said.
"We need decisive action to fight deviant ideas because they are the justification of terrorism. We are determined to fight terrorism in all its forms."
The member states promised to change laws to criminalise the financing and incitement of terrorism.
"Islam is the religion of moderation. It rejects extremism and isolation. There is a need to confront deviant ideology where it appears, including in school curricula. Islam is the religion of diversity and tolerance," the statement added.
Saudi Arabian Foreign Minister Prince Saud al-Faisal said the declaration was irreversible and that it was the responsibility of national governments to implement the measures.
Whether Muslim leaders can live up to these ambitious goals will depend partly on their own political will, and partly on whether the OIC can be turned into an instrument of change, our correspondent says.
There is a widespread feeling that the OIC - a body set up under Saudi auspices in the 1960s - is in need of rejuvenation.
There is also talk of giving it a new charter, more money, and a catchier name.