President Bush ordered that flags nationwide be lowered to half staff, and said, "The world has lost a champion of human freedom" in Pope John Paul II. He called the pope "humble, wise and fearless priest who became one of history's great moral leaders."
"A good and faithful servant of God has been called home, " Bush said. Bush was expected to travel to Rome for the pope's funeral.
Former President Bill Clinton said Pope John Paul II was a symbol of unity in a divided world.
"In speaking powerfully and eloquently for mercy and reconciliation to people divided by old hatreds and persecuted by abuse of power, the Holy Father was a beacon of light not just for Catholics, but for all people," Clinton said in a statement.
U.N. Secretary-General Kofi Annan stressed that the pope "was a tireless advocate of peace, a true pioneer in interfaith dialogue and a strong force for critical self-evaluation by the church itself."
Cardinal Karol Josef Wojtyla of Krakow, Poland, was elected the 264th leader of the Roman Catholic Church in 1978, at age 58. The first non-Italian to assume the papacy in 455 years, he took the name John Paul, following the lead of his predecessor, Pope John Paul I, who died after just 33 days as pontiff. In 2003, John Paul II marked the 25th year of his papacy.
In the 2,000-year history of the church, no other pope has traveled as far. Fluent in eight languages, John Paul II visited more than 100 countries on every continent but Antarctica, making a point of seeing not only world leaders, but those in hospitals, slums and prisons.
John Paul often stunned political leaders by setting diplomacy aside and speaking out on controversial issues. In March 2000, he apologized for mistakes committed in the name of the church over the past two millenniums, including the Inquisition, the Crusades and the persecution of Jews. In January 1998, he made a historic visit to communist Cuba, where his appeals for freedom of speech, human rights and the release of political prisoners were the first noncommunist public speeches since 1959.
His trip, the first to Cuba of any pope, revitalized the Catholic religion on the island nation after almost 40 years of repression, even prompting President Fidel Castro to lift the ban on Christmas celebrations.
The peaceful revolution John Paul sparked in Cuba was much like the ones he supported in Eastern Europe and his native Poland. As a cardinal, he secretly aided the anti-communist struggle by smuggling money and supplies to the Catholic underground in what was then Czechoslovakia.
After becoming pope, John Paul II wrote letters of counsel to Solidarity trade union activists in Poland, where the communist regime imposed martial law in 1981.
His visits to Poland fueled the eventual success of the Solidarity movement there, which many see as the precipitating event in the collapse of the communist bloc. Soviet leader Mikhail Gorbachev proclaimed that without the support of John Paul, communism would have been "impossible" to overthrow. John Paul insisted, from first to last, that the Solidarity revolution be strictly nonviolent. It was.