The 57-member Organisation of Islamic Cooperation has suspended Syria, saying the Muslim world "can no longer accept a regime that massacres its people".
A statement issued at the end of an emergency OIC summit in the Saudi holy city of Mecca said participants had agreed on "the need to end immediately the acts of violence in Syria and to suspend that country from the OIC".
The final statement said there had been "deep concern at the massacres and inhuman acts suffered by the Syrian people".
OIC chief Ekmeleddin Ihsanoglu told a news conference early Thursday the decision sent "a strong message from the Muslim world to the Syrian regime".
"This world can no longer accept a regime that massacres its people using planes, tanks and heavy artillery," he added.
It was "also a message to the international community stating that the Muslim world backs a peaceful solution (in Syria), wants an end to the bloodshed and refuses to let the problem degenerate into a religious conflict and spill over" into the wider region, Ihsanoglu said.
The move was welcomed by the United States as sending a "strong message" to President Bashar al-Assad's regime.
"Today's action underscores the Assad regime's increasing international isolation and the widespread support for the Syrian people and their struggle for a democratic state that represents their aspirations and respects their human rights," State Department spokeswoman Victoria Nuland said.
The emergency summit of the world's largest Islamic bloc opened late Tuesday with the suspension proposal put forward by a preparatory meeting of foreign ministers, a symbolic attempt to pile pressure on Damascus over its deadly crackdown on a 17-month uprising.
The move by the OIC, which represents 1.5 billion Muslims worldwide, is aimed at further isolating Assad's embattled regime, but its effect is seen as being largely symbolic.
The only voice against was that of Iran, a staunch friend of Damascus.
Syria was suspended from the Arab League last year over its clampdown on the uprising that Assad characterised as a plot by Western and rival powers to overthrow his regime.
About 40 heads of government from the Arab world, Africa and Asia took part in the summit, presided over by Saudi King Abdullah.
In a second statement called the "Mecca Pact," the participants proclaimed their support for "Muslim people who are oppressed like the Syrian people".
It underlined the summit's support for "the oppressed Muslim peoples... who face the combat aircraft and heavy guns of the regular armies as is the case of the Syrian people".
The statement backed cooperation between Muslim states, the fight against divisions between Muslims, promotion of "moderate" Islam and the "fight against terrorism and the thinking behind it".
OIC solidarity with Myanmar MuslimsAnother major issue addressed by the conference was the situation of Myanmar's Muslim Rohingya people, displaced by deadly sectarian violence, which the summit decided to take to the United Nations.
The OIC condemned "the continued recourse to violence by the Myanmar authorities against the members of this minority and their refusal to recognise their right to citizenship".
"The summit has decided to bring this matter before the General Assembly of the United Nations," it said in its final statement.
The OIC announced on Saturday before the summit that it had received a green light from Myanmar to assist displaced Rohingya.
It said Myanmar gave its agreement following talks in Yangon on Friday between a delegation from the pan-Islamic body and President Thein Sein on the "deplorable humanitarian situation in Rakhine state".
The delegation assured Thein Sein that Islamic humanitarian organisations were willing to provide aid to all residents of the strife-torn state.
Saudi King Abdullah decided Saturday to grant $50 million to the Rohingya, the country's news agency SPA reported, describing them as victims of "several rights violations, including ethnic cleansing, murder, rape and forced displacement".
Violence between Buddhists and Rohingya has left scores dead, with official figures indicating that 80 people from both sides died in initial fighting in June.
The entire state has been under emergency rule since early June with a heavy army and police presence.