The 83-year-old leader of the world's 1 billion Catholics appeared short of breath, and read his arrival speech in a badly slurred voice.
Early on in the speech, he lost his place and asked an aide to read most of the rest before finishing the last paragraph himself.
He corrected himself several times for stumbling on the words. On other foreign visits, aides have read parts of his speeches for the pope — but never his arrival text.
Slovakia's president, prime minister and cheering well-wishers waving small Vatican flags greeted the pope as his plane touched down at Bratislava's airport. Among the crowd were gay and lesbian activists protesting what they contend is a growing intrusion of the church in public and private life.
There was heavy security on the second anniversary of the Sept. 11 attacks. Five thousand police officers and 500 special guards were deployed nationwide to secure the frail pope and his entourage during the trip, his third visit to the ex-communist country and the 102nd foreign pilgrimage of his papacy.
Police with bomb-sniffing dogs swept through the church earlier this week, and authorities said they were investigating a death threat made against the pope. They would not elaborate.
"We have no worries because of it. We are convinced that the pope's visit will be secure," said Marian Gavenda, spokesman for the Conference of Slovak Bishops.
John Paul, who suffers from Parkinson's disease and crippling hip and knee ailments, has appeared increasingly infirm during recent public appearances.
At the airport, the pope remained seated in a throne-like chair wheeled by aides. It took him 20 minutes to get off the plane.
On the eve of his visit, he asked for prayers that it would be a success and help usher in "a renewed springtime of faith and progress" for Slovaks.
Hinting that the country's turbulent communist past would be a theme, the pope hailed it as "a land enriched by the witness of heroic disciples of Christ who have left eloquent prints of holiness in the history of the nation."
The church in this overwhelmingly Roman Catholic nation suffered intense persecution under communism, which ended in 1989.
Many priests were ordained in secret, and hundreds were imprisoned or sentenced to forced labor by a regime that confiscated all church property. On Sunday, the pope will beatify as martyrs a bishop and a nun who were jailed and tortured in the 1950s.
Organizers said they expected up to half a million of the faithful from Slovakia and neighboring countries to attend open-air papal Masses in the capital, Bratislava, the central city of Banska Bystrica and the eastern city of Roznava.
On Thursday evening, after audiences with the president and prime minister and their wives, John Paul was to pray at the cathedral in the western city of Trnava, known locally as "Little Rome" for the devotion of its residents.
Seven in 10 of Slovakia's 5.4 million people are Catholics, and although there were almost no billboards or posters promoting the pope's visit, there was an air of expectation.
"Under communism, I was a math teacher and I couldn't go to church. I'd be fired if they found out," said Elena Boncova, a 79-year-old retiree, bursting into tears at the memory.
"But now I go — and I'm really looking forward to the pope's visit."
Others, however, expressed resentment that the government has spent $2.1 million on the visit.
"The money they are spending could have been used in a much better way for more needed purposes, like schools," said Anton Komada, 82, describing himself as an atheist.
The pope first visited here in 1990, a year after communism crumbled in then-Czechoslovakia. He returned in 1995, two years after the Czech Republic and Slovakia became independent states. He visited Croatia and Bosnia this summer.
On his latest pilgrimage, he will continue his campaign reminding Europeans of their Christian roots, and offer support to Slovaks preparing to join the European Union next spring.
"In 1990, he encouraged us — and we needed it, because we were worried about our future," President Rudolf Schuster told the national news agency TASR.
"Now the pope comes to Slovakia ahead of EU membership and encourages us again," he said.