Pope Francis begged for forgiveness from sex abuse victims in Chile on Tuesday as he said he feels 'pain and shame' over the scandal.

In his opening address after landing for a week-long tour of South America, the pontiff addressed the controversy directly after another three Catholic churches were torched overnight, bringing the total number to nine in the past few days in protest at clerical sex abuse.

Pope Francis apologised for the 'irreparable harm' caused by perpetrators of abuse after meeting with Chilean President Michelle Bachelet and addressing lawmakers, judges and other authorities at La Moneda palace.

They interrupted him with applause when he said he felt 'bound to express my pain and shame' that some of Chile's pastors had sexually abused children in their care.

'I am one with my brother bishops, for it is right to ask forgiveness and make every effort to support the victims, even as we commit ourselves to ensuring that such things do not happen again,' he said.

Francis didn't refer by name to Chile's most notorious paedophile priest, Father Fernando Karadima, who was sentenced in 2011 by the Vatican to a lifetime of 'penance and prayer' for sexually molesting children. The Pope himself is criticised for appointing Juan Barros as Bishop of Osorno despite him facing accusations of covering up the abuse.

Anne Barrett Doyle, of the online abuse database BishopAccountability.org which last week released research showing nearly 80 Chilean priests have been accused or convicted of abuse, praised Francis for opening his visit with the apology. But she said Chileans expect him to take action against complicit church leaders.

'This is a crucial opportunity for Francis: With luck, he will not make the mistake of his brother bishops in underestimating the savviness and moral outrage of the Chilean people,' she said.

Despite the tensions though, tens of thousands turned out to cheer 'Viva Papa Francisco' along his route from the airport.

Earlier in the day the Pope landed amid heightened security following the spate of attacks on churches ahead of his arrival. Speaking to reporters on the papal plane he warned the world was on the brink of nuclear war. 'I think we are at the very limit. I am really afraid of this. One accident is enough to precipitate things,' he said.