Pope Francis sets out to defend the weak as hundreds of thousands gather for inauguration mass in St Peter's Square

Former cardinal looked slightly awkward as he greeted Zimbabwean tyrant Robert Mugabe

The conversion of Jorge Mario Bergoglio officially took place this morning. As the world watched St Peter’s Square, the former Archbishop of Buenos Aires became Pope Francis – the new leader of the world’s 1.2 billion Catholics and the first non-European pontiff for nearly 1,300 years.

At 8.50am, in front of 100,000 cheering pilgrims and 132 foreign delegations, he emerged from his temporary lodging at the Vatican hotel to take a tour through the cheering crowds in an open-topped Popemobile.

Under clear blue skies he criss-crossed the piazza flanked by 20 or so bodyguards, stopping frequently to greet the faithful and kiss babies. At one point he got out of the vehicle to bless a disabled man.

He then entered St Peter’s Basilica to don his vestments and stop by the tomb of St Peter before re-entering the vast piazza to begin the special mass that would see him installed as the 266th Pope.

He began his homily with the words: “Dear Brothers and sisters”, and went on to emphasise his commitment to the poor. “The Pope must …open his arms to protect all of God’s people and embrace with tender affection the whole of humanity, especially the poorest, the weakest and least important,” he said.

But as any Pope knows, things don’t always got to plan. Within an hour, in the process of greeting visiting heads of state in the basilica, the new Pontiff found himself shaking hands with President Robert Mugabe.

Francis looked awkward as he greeted the Zimbabwean tyrant, who has been banned from entering the EU since 2002 following accusations of decades of human rights abuses in which thousands have died. Mr Mugabe, a practising Catholic, is able to pass through Italy to attend religious ceremonies at the Vatican.

Britain sent a three-pronged delegation that comprised Baroness Warsi, the former Tory chairman, Ken Clarke, the Minister without Portfolio, and the Duke of Gloucester, who is 21st in line to the throne and was there on behalf of the Royal Family.

Lady Warsi – who is now a foreign minister – denied that Britain’s low-key representation was a reaction to Francis’s past comments that the Falklands had been “usurped” by Britain. “Francis made those comments in the past when he was the Archbishop of Buenos Aires,” she said. “Relations between the Holy See and the British Government are currently – following the last papal visit to the UK – the best they’ve been for a very long time.”

As tens of thousands headed for the Vatican, Rome went into lockdown, with hundreds of extra police on the streets and security at the city’s airports on high alert.

While the vast piazza of St Peter’s was less crowded than many had predicted, some commented on the “tranquillity” of the atmosphere. “Everyone seems relaxed and happy. It’s very different from the atmosphere say, at the beatification of John Paul II: that was unlike anything I’ve ever seen,” said Vatican watcher Ann Schneible of the Zenit Catholic news service.

Many pilgrims shouted “long live the Pope.” One of the hundreds of banners read: “Assisi is waiting for you”.

Today’s ceremony began just after 9am at St. Peter’s tomb, where the Ring of the Fisherman and the Pallium woollen vestment of the papal office was conferred on a new Pope. Communion was given by the deacons of the church square and by another 500 priests in St. Peter’s Square.

In keeping with his reputation for modesty and lack of pomp, Pope Francis has opted for a gold-plated silver ring, rather than the usual solid gold.

After, Francis dedicated much of his homily to our treatment of the dispossessed. But alluding to his conservative social agenda, the new pontiff, who is opposed to gay marriage and full rights for divorcees, he also called on “all those who have positions of responsibility in economic, political and social life” to be “protectors of creation”.

Vice President Joe Biden represented the US, as President Barack Obama prepared for his visit to Israel. German Chancellor Angela Merkel, European Commission President Jose Manuel Barroso, Jean-Marc Ayrault, the French premier and King Albert II and Queen Paola of Belgium, were also present.