THE leader of Scotland's 850,000 Catholics led the tributes to Pope Benedict following his surprise decision to resign on health grounds.

Cardinal Keith O'Brien's comments came as world leaders, both religious and political, praised the 85-year-old pontiff following his momentous decision which surprised even the Vatican hierarchy.

He said: "I was shocked and saddened to hear of the decision to resign. I know it has come after much prayer and reflection.

"I will offer my prayers for Pope Benedict and call on the Catholic community of Scotland to join me in praying for him at this time of deterioration in his health as he recognises his in-capacity to adequately fulfil the ministry entrusted to him."

First Minister Alex Salmond said the Pope's decision to step down because of advancing age was "selfless". He said: "The Scottish Government holds the Holy See in the utmost respect and is aware Pope Benedict's decision will be a great shock to the Catholic community in Scotland and internationally.

"We should respect the decision of His Holiness to pass on his ministry in a selfless gesture, on health grounds, in the best interests of the Church.

"Like many Scots, I remember with great fondness the resounding success of Pope Benedict's visit to Scotland in 2010 and the papal mass celebrated at Bellahouston Park. I wish him a very peaceful retirement."

David Cameron spoke of the Pope's visit to Britain in 2010, when the two met while Mr Cameron was leader of the opposition.

He said: "He has worked tirelessly to strengthen Britain's relations with the Holy See,.

"His visit to Britain in 2010 is remembered with great respect and affection.

"He will be missed as a spiritual leader to millions."

Labour leader Ed Miliband said Pope Benedict had made a "brave" decision, adding: "Our thoughts are with those who must make such a critical decision on behalf of millions around the world."

US President Barack Obama thanked Pope Benedict for his service "on behalf of Americans everywhere", and said: "Michelle and I wish to extend our appreciation and prayers to His Holiness Pope Benedict XVI.

"Michelle and I warmly remember our meeting with the Holy Father in 2009, and I have appreciated our work together over these last four years.

"The Church plays a critical role in the United States and the world, and I wish the best to those who will soon gather to choose His Holiness Pope Benedict XVI's successor."

German Chancellor Angela Merkel praised her countryman's role in encouraging dialogue with Churches, Jews and Muslims. She said his speech before the German Bundestag in 2011 was a high point.

Mrs Merkel said: "The decision to step down deserves my greatest respect. Many will understand even the Pope has to deal with the burden of age."

She added he was "one of the most significant religious thinkers of our time".

Leaders of other religions also spoke of their sadness and shock at the resignation. The new Archbishop of Canterbury, Justin Welby, said: "It was with a heavy heart but complete understanding we learned this morning of Pope Benedict's declaration of his decision to lay down the burden of ministry as Bishop of Rome, an office which he has held with great dignity, insight and courage.

"As I prepare to take up office I speak not only for myself, and my predecessors as archbishop, but for Anglicans around the world, in giving thanks to God for a priestly life utterly dedicated, in word and deed, in prayer and in costly service, to following Christ."

The Most Rev David Chillingworth, Bishop of St Andrew's, Dunkeld and Dunblane and Primus of the Scottish Episcopal Church, said: "Christians of all traditions will have heard of the resignation of Pope Benedict XVI with regret.

"He has been a distinguished holder of his office, widely respected for his scholarship and his spirituality.

"Pope Benedict has made a difficult personal decision which shows the mark of a humble servant of Jesus Christ. We wish him a peaceful and holy retirement."

The Chief Rabbi, Lord Sacks, said: "I was honoured to welcome Pope Benedict XVI to Britain on behalf of non-Christian faiths in 2010 and spend time with him during a visit to the Vatican in 2011. I saw him to be a man of gentleness, of quiet and of calm, a deeply thoughtful and compassionate individual who carried with him an aura of grace and wisdom. I wish him good health, blessings and best wishes for the future."