Pope Benedict XVI has begun a four-day trip to Portugal, during which he is likely to call for solidarity across Europe's struggling economies and urge people to uphold Christian values.

Tens of thousands of people lined the streets of the overwhelmingly Catholic country to welcome the pontiff, whose five-year papacy has been rocked by the recent child sex abuse allegations.

The visit has been organised by Carlos Azevedo, the auxiliary bishop of Lisbon. Bishop Azevedo said the pope would focus on Europe's economic crisis, which has hit Portugal hardest, along with Greece.

'The pope will encourage institutions to lend a hand and show solidarity during difficult times,' the bishop said.

'Europe needs to be awoken, there is a lack of strong values. If there was greater ethical conscience, we would not have arrived at the current economic disaster.'

Civil servants in the capital have been given the afternoon off today to attend an evening open-air mass with the Pope on the banks of the Tagus. The organisers expect up to 160,000 people to attend.

'We are going to experience this as a celebration,' said Catarina Vieira, one of dozens of blue-uniformed scouts helping to prepare the mass site.

Lisbon's public buses are flying the yellow and white Vatican banner and bus shelters have been decorated with posters of the pope with a red robe against a green backdrop to resemble the Portuguese flag.

The pope will travel aboard his bullet-proof Popemobile from the airport to the papal nunciature. Since dawn today, hundreds of police officers have been deployed along the 8km route and letter-boxes on the route have been taped over.

Ligia Aguilar, a local shop owner, said 'Just because this pope has a certain personality doesn't mean that I will stop being a believer. I would go to mass with any pope that comes to Portugal'.

The pontiff will be welcomed by President Anibal Cavaco Silva, days before the president is due to sign a bill into law that will allow same-sex marriage. This will make Portugal the sixth country to legalise the practice.

After the welcome, the pope is to pray in the chapel of the 16th century Jeronimos monastery, whose renowned religious carvings have made it a UNESCO World Heritage Site.

This evening's mass is one of three giant gatherings that have been scheduled for the visit. According to the Vatican, the 'heart' of the trip will be the pope's stop in Fatima, the site where three shepherd children reported seeing visions of the Virgin Mary in 1917.

The pope's visit to the sanctuary town comes exactly ten years after his predecessor John Paul II beatified two of the children at the site, bringing them one step away from sainthood.

Another open-air mass is to be held in the northern city of Oporto on Friday, shortly before the pope returns to Rome.