Benedict will be in Valencia for about 26 hours to close a Vatican-organized gathering on family issues, and he wasted no time in defending the Vatican's vision of marriage as a union of man and woman.
"The family is a unique institution in God's plan, and the church cannot fail to proclaim and promote its fundamental importance," Benedict said on his arrival.
In a city festooned with flags and balloons bearing the yellow and white colors of the Vatican and brimming with pilgrims from around the world, the pontiff also prayed foron Monday, a tragedy that added a tinge of sadness to his first visit here as leader of the Roman Catholic Church.
Even before arriving, the pope said on his plane that according to human nature, it is man and woman who are made for each other and give humanity a future, reports CBS News correspondent Sabina Castelfranco.
The pope is in Valencia for a meeting organizers predict will draw 1.5 million people to this Mediterranean port city.
Asked about the role of gay marriage in society, the pope told reporters: "According to human nature, it is man and woman who are made for each other and to give humanity a future."
King Juan Carlos, Queen Sofia and Prime Minister Jose Luis Rodriguez Zapatero welcomed the pope on his arrival and a military band played the national anthems of Spain and the Vatican as hundreds of faithful endured sweltering, muggy weather to see the pontiff.
The pope and the premier were to meet in the evening for what could be a dialogue of the deaf: besides legalizing gay marriage, Zapatero's government has made it easier for Spaniards to divorce and halted a plan by a previous, conservative government to make religion classes mandatory in public schools.
Spain has passed from being a bastion of Roman Catholicism to a predominantly lay society in less than a generation. Statistics show that while 80 percent of Spaniards still call themselves Catholics only 42 percent believe in God and 20 percent go to Mass.
Benedict visited the city's 13th century cathedral where he handed the head of Spanish Episcopal conference a letter addressed to the country's bishops. In the letter, the pope encouraged them in their activity at a time, he said, of rapid secularization. He urged bishops to continue to proclaim that doing without God or acting as if he did not exist undermines the truth about man and compromises the future of culture and society, Castelfranco reports.
Tens of thousands of faithful, waving Vatican flags, clapping and releasing yellow and white balloons, lined the streets of Spain's third-largest city for a glimpse of the pope as he drove by in his bullet-proof pope mobile.
Maria Luisa Galdon, a 54-year-old housewife with three daughters, said Spain needs the visit because families here are in trouble. "What's missing is love, understanding and education," she said.
She thinks the pope should give Zapatero a message to improve relations with the church and give it more support. "I think he will do it politely and necessarily," Galdon said.
Jose Ramon Gonzalvez, a 38-year-old civil servant who lives in the neighborhood of the subway crash, called the pope's stopover at the metro station "a very important declaration of his support for all of us and an expression of his love."
People in the crowd there chanted the pope's name with same rhythm used in the fight song of the Spanish national soccer team. They snapped digital photos of the pope and giggled as they showed them to each other.
The Vatican seemed irritated that Zapatero was not planning to attend a papal Mass on Sunday.
"The Holy See doesn't invite leaders to Mass," said papal spokesman Joaquin Navarro-Vals. But he noted that such left-wing stalwarts as Fidel Castro of Cuba and former Nicaraguan President Daniel Ortega have attended papal masses.