Illawarra's Islamic community has condemned the Martin Place siege as a despicable act that plays into the hands of those who seek to destroy the goodwill of Australians and ridicule Muslims.

"We reject any attempt to take the innocent life of any human being, or to instil fear and terror into their hearts," a coalition of Muslim groups said in a statement endorsed by Illawarra spokesman Gary Ismail.

"This despicable act only serves to play into the agendas of those who seek to destroy the goodwill of the people of Australia and to further damage, tar and ridicule the religion of Islam, and Muslims throughout this country."

NSW police don't know what motivated an "unstable" Man Haron Monis to take 17 hostages at a popular cafe but admit he had a long criminal history and held extremist views.

The crisis ended with the deaths of three people, including the self-described Muslim cleric.

Mr Ismail condemned the actions of the so-called Muslim and called for all Australians to unite and pray for the hostages and their loved ones.

The Illawarra People for Peace movement, chaired by Mr Ismail and Catholic priest Father Ron Peters, will hold a multi-faith prayer meeting on Friday morning to show support for the people in the siege.

The religious leaders launched the movement earlier this year to promote peace and harmony in the region and beyond.

Father Peters said the small initiative in the Illawarra "is so significant in bringing to mind to all people that peace and harmony are desired by all people who simply want to live life to the full".

"This awful series of events in Sydney reminds us of our call to promote mutual respect and to live in friendship - no matter the faith or the non-faith of the individual or community," he said.

"We need to understand and appreciate that such an event needs sensitive planning by all persons concerned, especially the police and the local council."

Father Peters said: "We pray for the two victims and their families and friends.

"We pray for those people traumatised by the siege and their families and friends. We are so grateful to the Police Command of NSW, the police and the tactical response men and women, as well as our Premier for his sterling leadership of our beautiful state.

"We hold the people of NSW and Australia in prayer with the specific intention that we respect and build relationships for the greater glory of God."

The peace event will be held in Crown Street Mall from 10.15am.

Last night, St Michael's Catholic Church in Thirroul held special prayers for the victims, emergency service workers and their families and friends.