Rebels won Syria's long-vacant seat at the Arab League on Monday, on the eve of the organisation's summit in Doha, despite rifts within the opposition that have marred their political gains.
A high-ranking League official in the Qatari capital told AFP that the opposition National Council "has been invited to the Arab summit and will occupy Syria's seat" at the 22-member Arab League.
Damascus reacted with fury at the announcement.
"The League has handed Syria's stolen seat to bandits and thugs," Syrian official daily Al-Thawra said.
"They have forgotten that it is the people who grant the powers and not the emirs of obscurantism and sand," it said, in an apparent reference to key opposition supporters Qatar and Saudi Arabia.
A state television station said: "Qatar wants to bypass the rules of the Arab League by giving the seat of a founding member of the League to a coalition that obeys only the money and fuel of the Gulf and submits to American dictates."
The news came a day after National Coalition leader Ahmed Moaz al-Khatib announced his resignation, throwing the fragmented opposition into disarray and denting its credibility.
However, Khatib said he will address the summit "in the name of the Syrian people," while the Coalition's envoy to Doha, Nizar al-Haraki, told AFP that Khatib will head the delegation representing Syria on Tuesday.
"I have decided to make a speech in the name of the Syrian people at the Doha summit," Khatib announced in a statement on his Facebook page, saying he took his decision after prayers and consulting friends.
"This is not linked to the resignation which will be later discussed," he added.
The coalition has said it refuses Khatib's resignation.
Haraki told AFP that Khatib will "head the eight-member Syrian delegation at the summit and will occupy Syria's seat." The delegation will include Syria's first rebel prime minister, Ghassan Hitto.
The Arab League on March 6 called on the coalition "to form an executive body to take up Syria's seat" and attend the summit, although Iraq and Algeria have expressed reservations, while Lebanon has distanced itself from the decision.
The League in November 2011 suspended Syria after Damascus failed to implement an Arab deal designed to end violence against protesters.
The move came after President Bashar al-Assad's regime launched a bloody crackdown on dissent which has since morphed into a civil war in which more than 70,000 people have been killed, according to UN figures.
National Coalition member Ahmed Ramadan said Khatib has come under "intense pressure from Arab foreign ministers and from within the Coalition to reconsider his decision."
Key opposition backer Qatar has also urged Khatib to reverse his decision, which came just days after Hitto's election in Istanbul.
An opposition source told AFP that Khatib has accused "certain countries, notably Qatar, of wanting to control the opposition" and of having imposed Hitto as premier.
But Haraki said Monday that "what's between him (Khatib) and Hitto is a difference in opinions, not a dispute."
A Syrian opposition member, who requested anonymity, told AFP that Khatib's resignation was "a desperate cry to protest the international failure to fulfill promises for aid."
Khatib was accusing Qatar of having imposed the election of Hitto against Saudi-backed candidate Imad Mustafa, the same source said.
In eastern Syria, rebel Free Syrian Army commander Riad al-Asaad was wounded on Sunday night in a blast that hit his car, the Syrian Observatory for Human Rights said.
A government official in Ankara confirmed Asaad had been hurt, saying he had lost a leg in the attack but was in "good condition" after being rushed across the Syrian border into Turkey for treatment.