The Arab League has voted to suspend Syria from its meetings and impose sanctions against Damascus over its failure to end a government crackdown on protesters.
It asked member states to withdraw their ambassadors, and urged Damascus to end violence against protesters.
The vote came after Syria ignored an Arab League proposal envisaging the start of dialogue with the opposition.
But Syria's representative said the decision violated the league's charter.
Youssef Ahmed told Syrian state TV said it showed the league was "serving a Western and American agenda".
In London, British Foreign Secretary William Hague said: "I welcome the strong stance taken by the Arab League... The continuing violence is deplorable and must stop".
The Arab League proposals - accepted by the government of President Bashar al-Assad - include the release of prisoners, the withdrawal of security forces from the streets and talks between the government and opposition.
But the violence has continued, with the city of Homs bearing the brunt, say human rights activists. Twelve died on Saturday.
US-based Human Rights Watch (HRW) issued a report this week documenting allegations of torture and unlawful killings in the city, and called on the Arab League to step up pressure on Damascus.
President Assad has sought to put down the protests since March. The UN says more than 3,500 people have died in the protests so far.
'Concern for Syria'
Eighteen Arab League member states voted at the Cairo meeting to suspend Syria, with Syria, Lebanon and Yemen voting against and Iraq abstaining.
The BBC's Jon Leyne in Cairo says the decision is the most that anyone could have realistically expected from the Arab League.
It is a huge blow to Syria's pride, and could also be a real practical blow to its leaders, our correspondent adds.
But opposition groups are already calling for more action, he says, including a no-fly zone.
Qatari Prime Minister Sheikh Hamad Bin Jassim said the suspension would take effect on Wednesday, adding that talks would be held with Syrian opposition groups in three days' time.
"We were criticised for taking a long time but this was out of our concern for Syria," he said, quoted by Reuters news agency.
"We needed to have a majority to approve those decisions.
"We are calling all Syrian opposition parties to a meeting at the Arab League headquarters to agree a unified vision for the transitional period."
Mr Jassim said that if the violence and killing did not stop, the Arab League would contact human rights organisations and the UN and "set a vision for the appropriate measures to stop that bloodshed".
This vision would be submitted to another league meeting on 16 November, he added.
The Arab League's condemnation of violence by Col Muammar Gaddafi's government in Libya and call for a no-fly zone paved the way for a UN Security Council resolution protecting civilians and a Nato-led mission there.
But Mr Jassim denied that Saturday's decision would lead to international intervention in Syria.
"No-one is talking about a no-fly zone, people are trying to mix up the cases. None of us is talking about this kind of decision," he said.
Violence continued in Syria on Saturday with 12 people killed, the Local Co-ordination Committees said - four of them were in Deraa and three each in Homs and Idlib.
Thirteen people died on Friday, most of them in Homs.
There were also reports of violence and mass arrests in the capital, Damascus.
Mass street protests after Friday prayers, followed by brutal crackdowns by security forces have become a weekly feature of Syria's uprising.
President Assad's government insists it is battling armed gangs and militants and says hundreds of soldiers and police have been killed.
The government has restricted foreign journalists from entering the country, making it difficult to confirm events on the ground.