Speaking at a conference organised by the Brookings Doha Centre in the Qatari capital, former Muslim Brotherhood leader Ali Al-Bayanouni said the group was "not imposing itself on the opposition or on the people of Syria."
"We support the establishment of a modern, civil, democratic state," he said.
Bayanouni said the newly formed Syrian National Council represented "80 per cent of the Syrian opposition."
Opposition groups, both Islamist and secular, announced the council's formation at a meeting in Istanbul on Sunday, vowing to forge a united front against Assad's regime.
The Brotherhood's current leader, Riyad Al-Shaqfa, was appointed to the council's board.
Al-Bayanouni criticised the international community, particularly the United States, for failing to take a stronger stand against Assad's brutal crackdown on anti-government protesters.
"We hope the US finally realises that its interests lie with the people and not with the regime," he said.
The Muslim Brotherhood is banned in Syria and all of its officials live in exile.