Thousands of schoolchildren and their teachers are travelling to west London to see this morning's address by the Pope at his "Big Assembly".
Some 3,000 children will hear Pope Benedict XVI address them at a gathering at St Mary's University College in Twickenham.
The Pope is on his second day of a four-day state visit to the country and the first by the head of the Catholic Church since John Paul II made a pastoral visit back in 1982.
Those who cannot attend the event, which is due to finish at midday, will be able to watch the address via a live feed on the Catholic Education Service's website while hundreds of children from Catholic schools in York will watch the event on giant screens at the city's racecourse.
Many children and teachers came to the capital yesterday. Among them was Jesus Echevarria, head of the 500-pupil St Augustine's Secondary in Scarborough, who made the trip with two teachers and six children, aged nine to 11.
Mr Echevarria, who studied at the college which is hosting this morning's event and missed Pope John Paul's visit in order to study for accountancy exams, said the children had been chosen to go from a number of criteria, including who would be good ambassadors.
"We knew we couldn't take the whole school," he said. "They're going to be mixed with a whole load of other children, which will be a good experience," he added. "They are really looking forward to it."
The Pope will lead the congregation in the Lord's Prayer while two schools - the Holy Rosary and St Anne's Catholic Primary and the Maria Fidelis RC Convent School in north London - will sing for him and lead the congregation in three hymns.
Maria Fidelis assistant head Lyn Newell said: "We were told not to say anything because of the security issues. Because we're on stage, the security is like getting on an aeroplane."
John Bamford, a 15-year-old pupil at All Saints RC School in York, whose school chaplain Father Stephen Maughan met the current Pope while studying to be a priest in Rome, admitted he didn't know why he'd been chosen as one of the five children from his school going to the event.
"I was told about two weeks ago," he said. "I go to Lourdes every year with school but I'm pretty excited I'm going to this. I hope I'll get to meet him."
As part of the event, the Pope will also be presented with a gift of a cross by pupils from the Holy Cross Catholic School in Plymouth who have been chosen to attend following deputy head Leah Burch's work to provide materials and teaching help for St John Vianney School in Gambia. A representative from the African school will also be at the event and Mrs Burch said the 30-strong team from the school had left at 5.45am yesterday to arrive in time for rehearsals.
Around Britain in four days
The Pope was greeted by the Queen yesterday in Edinburgh, ahead of holding an open-air mass in Glasgow. He is scheduled to meet the Archbishop of Canterbury later today before giving a state address at Westminster Hall.
His itinerary on Saturday includes visiting a care home for the elderly in south London and an open-air event in Hyde Park. He will end his visit by meeting the bishops of England, Scotland and Wales in Birmingham on Sunday before travelling back to Rome later that day.