Opposition chief Ahmed Moaz Al-Khatib took Syria's seat at the Arab League on Tuesday as leaders gathered for the annual summit in Doha gave member states the "right" to arm rebels in the war-torn country.

The summit affirmed the "right of every state to offer all forms of self-defence, including military, to support the resistance of the Syrian people and the Free Syrian Army," a resolution said.

However, "efforts aimed at reaching a political solution to the Syrian crisis are a priority," it added.

Launching into a fiery speech after leading an opposition delegation into the Doha meeting to thunderous applause from Arab leaders, Khatib also demanded that the opposition be allowed to represent Syria at the United Nations.

Khatib, who threw the opposition into disarray by announcing his resignation on Sunday, made it clear that he was still firmly at the helm of the Syrian National Coalition, the main opposition umbrella grouping.

Taking the seat at the invitation of Qatar's emir, Sheikh Hamad bin Khalifa Al-Thani, Khatib was flanked by other senior opposition figures, including newly elected rebel prime minister Ghassan Hitto.

"We demand... all forms of support from our friends and brothers including our full right for self-defence and the seat of Syria at the United Nations and at other international organisations," he said.

He called for a "freezing of the funds of the regime which it stole from our people," estimated by the opposition at around $2 billion.

He also stressed that the Syrian people alone would determine the future of their country.

"They ask who will rule Syria. The people of Syria will decide, not any other state in this world," Khatib said in an apparent allusion to the influence of summit host Qatar and its heavyweight neighbour Sadui Arabia over the opposition.

The seat has been empty since Syria was suspended in November 2011 after Damascus rejected calls to end violence against protesters and instead pressed a bloody crackdown on dissent.

Protests escalated into an armed rebellion against Assad's regime and later into a civil war in which the UN says more than 70,000 people have been killed.

Damascus reacted furiously to the decision.

"Shame on you, Arab brothers," wrote state-owned daily Tishreen.

"This theft that the sheikhdom of Qatar and other collaborator, treacherous, backward Arab regimes have committed by handing the Doha-sponsored Coalition the Syrian state's membership... is a legal, political and moral crime," it said.

Regime supporters hacked the Arab League website to protest the decision.

They accused the league of having "crowned its allegiance to the mini-gas state of Qatar, Israel's ally, by handing the seat of the Syrian Arab Republic... to this illegitimate dwarf called the Coalition."

Khatib made no mention of his resignation and said ahead of the summit that it would be dealt with after the meeting is over.

Spokesman Khaled al-Saleh said the resignation "has not been accepted" and that "most members of the coalition want him to continue to lead" at this time.

But Khatib pointedly expressed his confidence in Hitto.

"We trust him. The general assembly of the coalition awaits his programme to debate it," he said.

However, a group of prominent opposition members criticised the role of the Muslim Brotherhood in the Coalition and accused regional governments of "hegemony" over the body.

In a letter to the Arab League, they said "the crisis in the Syrian opposition is worsening... (because of) what is happening inside the National Coalition and the actions of those who dominate it."

Although the letter made no explicit reference to the Brotherhood, it came amid discontent in the opposition over the election of Hitto, widely understood to have been the Brotherhood's choice.

Meanwhile, Khatib said he had asked US Secretary of State John Kerry to extend the protection of Patriot missile batteries deployed along the Turkish border into northern Syria, and that Kerry had "promised to look into the matter."

"We are still awaiting a decision from NATO on this matter," Khatib said.

But White House spokesman Jay Carney responded by saying "we are aware of the request" but "at this time, NATO does not intend to intervene militarily in Syria."