Pope Benedict XVI arrived for a week-long celebration of youth and religion yesterday with a promise to apologise to Australian victims of sexual abuse in the Church.

Benedict, 81, hopes that the longest trip of his Pontificate, which culminates in Sydney's World Youth Day celebrations, will reinvigorate the Australian Catholic church.

Even as he was welcomed by Kevin Rudd, the Prime Minister, local Catholic leaders sought to minimise the fallout from a sex abuse case and criticism of tough new police powers to protect pilgrims from “annoyance” by critics of the Church.

During his flight from Rome, the pontiff said he would apologise to sex abuse victims, as he did during a visit to America in April. “We have to see what was insufficient in our behaviour and how we can prevent, and heal, reconcile,” he said. “It must be clear, being a priest is incompatible with this behaviour because priests are in the service of Our Lord.”

His comments echoed repeated references to the shame that paedophile priests had wrought on the Church during his US visit. The Church has been criticised for moving abusers between parishes rather than defrocking them or reporting them to police. Leaked documents last week deeply embarrassed the Pope's friend and Australia's most senior Catholic, Cardinal George Pell, who admitted to a mistake in a “badly worded” letter written in 2003 to a man who claimed to have been sexually assaulted by a priest in 1982. The Cardinal wrongly told him an inquiry had not upheld his complaint.

Cardinal Pell has been forced to admit he may have been wrong to believe the priest's claim that the sex had been consensual, and has opened a new inquiry into the abuse. Campaigners say there have been 107 convictions for sexual abuse within the Church in Australia but say that this masks the true scale of the problem.

The Broken Rites pressure group said that the papal apology needed to be specific and meaningful. Bernard Barrett, a spokesman, said: “We want the Pope to force Cardinal Pell and all Australian bishops to be more transparent and less evasive when handling complaints from victims.”

On his way to Australia the Pope also expressed his intention to “wake up consciences” on climate change. He said: “We have responsibilities towards creation.” His is a topical message in a nation watching its greatest river system die in the worst drought on record.

The Pope will spend his first three days in Australia at an Opus Dei retreat. On Thursday he will visit the tomb of the Australian nun Mary MacKillop. Benedict's predecessor declared her “blessed” in 1995 and she now needs official confirmation of a second miracle to become Australia's first saint. He will also take a Sydney Harbour boat trip, a helicopter ride and meet disadvantaged young people and Aborigines. The celebrations incorporate old and new - from a collection of saintly bones to daily papal text messages and 165 rock, rap and jazz concerts. The highlight will be a Mass on Sunday at the city's racecourse.

The Pope's focus will be on the under-35s, who he believes hold the key to renewal of a church “in crisis”. He will urge them to embrace the Holy Sprit and push back the “tide of secularism”.

The octogenarian faces a challenge connecting to youth, especially in Australia, which he previously singled out as a nation where the Church was dying. In the decade to 2006 the number of Christians fell from 71 per cent to 64 per cent. While 5.1 million Australians claim to be Catholic, no more than 800,000 regularly attend church. The priesthood is in decline and ageing, with numbers down 20 per cent since 1971, and the average age up from 44 to 60. Only 141 were in training in 2005, a quarter of the number in 1969.

In hedonistic Sydney, home to one of the world's biggest gay and lesbian Mardi Gras, not everyone is sympathetic. While the New South Wales government has extended police powers to arrest people for “causing annoyance” to World Youth Day participants - with fines up to $5,500 (£2,700) - that hasn't stopped locals printing shirts with messages such as “Pope go Homo”. Bishop Anthony Fisher, an organiser, said the hostile reception was confusing pilgrims. “It's a pity that sometimes there's been a lot of negativity in the air - some of our pilgrims are saying, ‘What's going on here? This is the most wonderful thing for your country and for your church, they should be happy',” he said.

There was a carnival atmosphere in Sydney with footpath confessionals and music, and organisers claimed to be on track for 125,000 foreign “pilgrims” as part of a total attendance bigger than the 2000 Sydney Olympics.

I'd just like to say how sorry I am ...

— On a trip to America in April, Pope Benedict spoke on four occasions of his shame and sorrow at sex abuse by Catholic priests in the US. The scandals cost the Church $1 billion in compensation claims

— At the Vatican in March 2000, Pope John Paul II made a direct “plea for forgiveness” to God over the past sins of the Catholic Church, including the Crusades, forced conversion and its treatment of the Jews

— After prompting fury among Muslims in a speech in which he quoted a medieval Christian emperor, Pope Benedict stopped short of a full apology. The Vatican delivered a statement saying he was “very sorry that some passages of his speech may have sounded offensive to the sensibilities of Muslim believers”

— In 1995 Pope John Paul II wrote a “Letter to Women” in which he admitted that women had often been marginalised and reduced to servitude. “If objective blame,” he wrote, “has belonged to not just a few members of the Church ... I am truly sorry”.

Sources: Times Archives, vatican.va