Thousands of mourners packed Sydney's St Mary's Cathedral on Thursday for the controversial figure's requiem mass, while others gathered outside to watch the service on screens.

The former Catholic archbishop of Melbourne and Sydney will be laid to rest after the mass, following his death in Rome last month aged 81.

The mass is being celebrated by Sydney's Catholic Archbishop Anthony Fisher, who paid tribute to his friend, the son of a Ballarat publican, who spent 56 years as a priest and rose to become the third most powerful man in the Catholic Church.

Cardinal Pell was a bold and brave advocate for the Catholic Church who brought World Youth Day to Australia and "remained very much an Australian until the end", Archbishop Fisher told the congregation.

He said the "giant of a man" had a big heart, and that when he was ordained at the Vatican he adopted the motto "Be not afraid".

The 12 books he authored included journals he wrote while spending 404 days in prison after being convicted of molesting two teenage choirboys in 1996. The convictions were later quashed.

"Even after he was unanimously exonerated by the High Court, some continued to demonise him," Archbishop Fisher said.

"But many appreciate the legacy of this most influential churchman in our nation's history."

NSW Police riot squad units remain outside the cathedral to ensure a buffer zone is maintained between mourners and protesters, who marched past the cathedral forecourt chanting "George Pell go to hell".

The Vatican ambassador to Australia, Charles Balvo, read a message from the Pope during the service thanking his "faithful servant" for his work reforming the Church's finances.

He said that as Vatican treasurer Cardinal Pell diligently implemented "economic reform for which he laid the foundations with determination and wisdom".

Opposition Leader Peter Dutton, former prime ministers John Howard and Tony Abbott, media personality Alan Jones and divisive politicians Mark Latham and Matt Canavan are in the congregation

Prime Minister Anthony Albanese, NSW Premier Dominic Perrottet and Opposition Leader Chris Minns are not attending.

An 11th hour compromise was reached on Wednesday to allow a protest organised by LGBTQI group Community Action for Rainbow Rights to proceed.

Protest organisers agreed not to march on College Street directly next to the cathedral, but were allowed to gather on the other side of the road, within earshot of mourners.

Police clashed briefly with protesters as they removed a sign claimed to be unlawfully fixed to a point across the road from the cathedral.

"We're not here to cause trouble, we're here to bring attention to the abuse, that's all," protester Max Elbourne told AAP.

Rally organiser Kim Stern said the protesters would abide by police directions as they chanted and heard speeches by LGBTI and abortion rights activists.

"We want a strong, loud, vibrant and visible rally to oppose everything Pell stood for," he said.

Thousands of colourful ribbons attached to the cathedral's gates in memory of those who suffered sexual abuse by clergy were removed overnight.

Tensions flared briefly about 9am on Thursday when several mourners tried to remove ribbons tied to a small area of cathedral fence where the Church had allowed them to be tied.

Cardinal Pell's conservative stance on issues including gay marriage and abortion rights had been the subject of criticism, and he faced allegations of covering up child abuse.

Australian Associated Press