Cardinals set for a conclave this week to elect a new leader for the world's 1.2 billion Catholics celebrated masses across Rome on Sunday, as parishioners picked their papal favourites.

Some of the 115 cardinals who will choose a successor to Benedict XVI addressed the faithful before they are shut away from the world in the Sistine Chapel on Tuesday to make their choice.

Cardinal Odilo Scherer, the charismatic Brazilian whose stock is rising in a race without a clear favourite, attracted a packed congregation at the Sant'Andrea al Quirinale church.

The multi-lingual Archbishop of Sao Paulo said the week of meetings that the cardinals have held before the conclave -- expected to have included the scandals which rocked the Church during Benedict's papacy -- had filled him with joy.

"The atmosphere was very brotherly. It was a week which was truly conducted in a collegial spirit and in a spirit of responsibility for all of the Church, and I am delighted by that," Scherer said.

US Cardinal Sean O'Malley, who is also seen as a possible contender for the papacy, said in his homily that the Catholic world was "united in prayer" as the clock down ticked to the conclave.

"Let us pray that the Holy Spirit enables the Church to choose a new pope who will confirm us in our faith and make more visible the love of the Good Shepherd," O'Malley told parishioners in Santa Maria della Vittoria church.

In an apparent reference to the rising secularism which is one of the key challenges for the Catholic Church, O'Malley based his homily on the Biblical parable of the Prodigal Son, who runs away from home but is welcomed back with open arms by his father.

"People leave the Father, the Church, for many reasons -- ignorance, a poor welcome, negative experiences, scandals, spiritual mediocrity," he said.

Father Stefano Guernelli, rector of the ornate Baroque church in central Rome, told O'Malley: "I hope this is the last time you come here as a cardinal and I hope that if you are elected pontiff this will be the first church that you visit."

Another favourite to succeed the first pope to retire for 700 years has emerged among seasoned Vatican watchers -- Italian Cardinal Angelo Scola, a conservative theologian.

Scola celebrated mass in the Church of the Twelve Holy Apostles, surrounded by dozens of cameramen and photographers.

"I think he is a good man, he would be a fine leader to help strengthen the Church. I am praying for him," said 69-year-old parishioner Maria Bettini.

But hairdresser Giuseppina Fazzo, 47, was not convinced.

"I don't know whether Scola for pope is a good idea, we need a radical change in the Church and I don't know if he is the man for the job," she said.

At the nearby basilica of Santa Pudenziana, Rome's Philippine community gathered in their hundreds as they do every Sunday and sang the praises of Cardinal Luis Antonio Tagle, seen as another "papabile", or papal candidate.

"We are all maids here," said Meliros Gangani, 53, who came to Italy 26 years ago and works as a volunteer at the church on her days off.

"We really hope Tagle wins. If he does we would all feel a lot more confident with our bosses. He would really help unite the community," she said.

At the Chiesa della Nativita, or Church of the Nativity, the mass had an African flavour and hopes were high that the conclave could shun another European pope and instead choose the first pontiff from Africa in the modern age.

"It would be a strong sign," said Justin Golo, a Congolese priest. "Europe just really doesn't have a lot to offer (the Church) at the moment.

"In Africa, we are really living the faith and the churches are full. It is essential that the pope comes from a country where the faith is still alive."

Peter Turkson of Ghana has been mentioned as a possible African pope, but he has faced criticism after he screened a controversial video on Muslim immigration at a synod.