A large crowd turned up at Sydney airport to farewell Sydney's most senior Islamic cleric, who was deported on Sunday night.

Sheikh Mansour Leghaei, a Shia cleric, was ordered to leave Australia after being declared a security risk.

Dr Leghaei boarded a flight to Iran on Sunday night and about 200 people from the Islamic Youth Centre at Earlwood turned up to support him.

He has lived in Australia for 16 years and says his deportation is unjustified.

Dr Leghaei says he has never been given an explanation as to why ASIO has deemed him a security risk.

"Whatever they think and whatever they may have is absolutely wrong," he said.

"There's an error somewhere. We tried our very best to help them and correct that error.

"They didn't let us, simply because the act does not allow us to do so."

Under Australian law, ASIO is under no obligation to tell Dr Leghaei why he is under suspicion.

Dr Leghaei's supporters say he is a leader of inter-faith relations between Muslims and Christians.

The United Nations are said to have written to Australia's Immigration Minister asking that Dr Leghaei not be deported, but the request was denied.

Dr Leghaei had planned to fly out with his wife and daughter but leave behind his three sons.