John Paul II's body was displayed to the world on Sunday, his face showing signs of prolonged physical suffering.
"He died with the serenity of the saints," Cardinal Angelo Sodano told a huge crowd assembled for a sombre requiem mass.
The Pope's corpse, clad in crimson and white vestments, was put on view for the world by Vatican TV. He lay on a bier under a simple crucifix with his bishop's staff under his arm.
About 200,000 worshippers gathered in St Peter's Square to hear the pope's last message read at the Mass.
"It is love which converts hearts and gives peace," said the text, which was prepared for the Sunday after Easter and was read out by an archbishop.
His body is expected to be transferred to St Peter's Basilica at 5pm (1500 GMT) on Monday for public viewing, and his funeral is to be later in the week, possibly on Friday.
Cardinals will meet on Monday morning to fix the timetable, with more than 100 world leaders expected to attend the funeral, including US President George Bush.
News of his death has set off one of the greatest influxes of pilgrims in Rome's history.
Pilgrims are flocking to Rome to
pay tribute to John Paul II
"He has called us and we have come," Giuseppe Incarnati, who rushed to the tiny Vatican City from Naples to be close to the deceased Pope, said.
Within 15 to 20 days after the death, 117 cardinals aged under 80 will meet behind closed doors in the Vatican's Sistine Chapel to decide a successor.
When they elect the next pope, white smoke will pour from the chapel's chimney.
World leaders hailed John Paul as a force for peace during his 26-year papacy, while others credited him with a major role in the fall of the Iron Curtain.
Cardinal Sodano, one of his potential successors, called him John Paul the Great, joining others who have suggested he become only the third pope in two millennia to have such a title.
But liberal Catholics criticised his proclamations against contraception, abortion, married priests and women clergy.
"His pontificate was full of contradictions. The direction in which he took the church internally was very distressing for those who had hopes for real reform"
We Are Church Catholic reform movement
"His pontificate was full of contradictions," said the We Are Church Catholic reform movement. "The direction in which he took the church internally was very distressing for those who had hopes for real reform."
But in his native Poland, dissenting voices were hard to hear amid scenes of nationwide mourning.
More than 100,000 worshippers packed the central square in the capital, Warsaw, while 60,000 gathered in Krakow, where he was archbishop from 1964 until he became pope in 1978.
From Brazil to the Philippines, South Africa to Germany, Roman Catholics prayed and mourned. Many countries decreed periods of national mourning.
Many Muslims round the world praised John Paul for having pressed to build bridges with Islam and said his death had cost both faiths a campaigner for peace and justice.
Muslims praised the Pope for the
bridges he built with Islam
The pope led a campaign over the past two decades to help turn standoffs between the world's 1.1 billion Catholics and 1.2 billion Muslims into cooperation.
He was the first pontiff to officially visit a mosque during a trip to Syria in 2001.
In Pakistan, Hafiz Husain Ahmad, of the Islamic Mutahida Majlis-e-Amal alliance, said the world had lost a man of peace.
"George Bush's talk of a crusader war was a clear negation of Pope John Paul's efforts to promote interfaith dialogue and harmony," he said.
In the Middle East, Israelis and Arabs united in mourning for John Paul, hailing him as a man of peace who sought to heal ancient wounds and forge a brighter future for the Middle East.
At the Church of the Nativity in Bethlehem, the traditional birthplace of Jesus, the faithful lit candles next to a portrait of the pope.
Worshippers in Nazareth, the Galilee town where Jesus grew to manhood, filled the Basilica of the Annunciation.
In Jerusalem, hundreds of worshippers chanting hymns marched by candle light in the pouring rain through the old walled city to Gethsemane, the site where Jesus was believed to have been arrested before his crucifixion.
"George Bush's talk of a crusader war was a clear negation of Pope John Paul's efforts to promote interfaith dialogue
Hafiz Husain Ahmad,
Pakistan's Islamic Mutahida Majlis-e-Amal alliance
Israeli Prime Minister Ariel Sharon called John Paul "a friend of the Jewish people" and said the world had lost "one of the most important leaders of our generation".
He said the pope had "worked to bring about historic reconciliation" between the Roman Catholic Church and the Jews, and the establishment of diplomatic relations between the Vatican and Israel in 1993.
Palestinian President Mahmud Abbas described John Paul as "a great religious figure who devoted his life to defending the values of peace, freedom, justice and equality for all races and religions, as well as our people's right to independence".
Lebanon, which has more than a million Catholics, most of them Maronites, declared three days of official mourning.
In the mountains of northern Iraq, followers of the ancient Chaldean Christian sect, watched over by guards armed with AK-47 assault rifles, gathered for Mass to mourn the Pope.
"This news touches me greatly," worshipper 26-year-old Wamibh Yuhana said. He said one lesson Iraqis could learn from the Pope was that he had forgiven the Turk who shot him in St Peter's Square in 1981.
Egypt's Grand Shaikh Muhammad Sayyid al-Tantawi applauded his "moderate position supporting Arab issues in each of Palestine and Iraq".
But not all in the Middle East were sympathetic. In Saudi Arabia, the birthplace of Islam, Abd al-Rahman al-Mashari, a 45-year-old engineer, said: "He meant nothing to me. He was not even as important as a hair on my head."