Reaching across a nearly 1,000-year divide, Pope John Paul II offered a sweeping statement of regret Friday for "sins of action and omission" against Orthodox Christians including the sacking of the ancient center of Greek Byzantium.

The declaration -- long awaited by Orthodox leaders -- could provide a new foundation for the pope's attempt to encourage dialogue between the two estranged branches of Christianity.

Especially important to Orthodox ears was clear remorse for the "disastrous" sacking of Constantinople by Crusaders in 1204 that contributed to the collapse of the Byzantine Empire about three centuries later. The fall of the city -- now Istanbul, Turkey -- is one of the bitter memories that have poisoned relations between the two churches.

The setting for the pope's comments also maximized their sway. The Greeks are widely considered a pillar of faith for the world's more than 200 million Orthodox. Any Greek Orthodox overtures to the Vatican would likely encourage other churches to follow.

No senior members of the Greek Orthodox Church were at the airport to greet the pope, underscoring the delicate and potentially tumultuous tone of the visit.

The 80-year-old pope, using a cane, was saluted by an Air Force honor guard. Two children from Greece's small Roman Catholic community held a bowl filled with Greek soil for the traditional papal kiss -- a ceremony that was in doubt over worries it could enrage Orthodox zealots opposing the visit.

The pope is beginning a six-day pilgrimage to retrace the steps of the Apostle Paul. The trip also includes visits to Syria and Malta.

John Paul's statement of regret came in a meeting with Greek Orthodox leaders several hours after he arrived in Athens.

"It is tragic that the assailants, which set out to secure free access for Christians to the Holy Land, turned against their own brothers in the faith," the pope said. "The fact that they were Latin Christians fills Catholics with deep regret."

The pontiff prayed for God's forgiveness for "the occasions, past and present, when sons and daughters of the Catholic Church have sinned by action or omission against their Orthodox brothers and sisters."

The head of the Greek Orthodox Church, Archbishop Christodoulos, clapped and smiled as the pope spoke. "The pope was very kind to us," he said. But then noted: "Much work to be done."

The pope's lament on the past avoided one of the most profound problems of the present: the Eastern Rite churches that follow many Orthodox traditions but are loyal to the Vatican. Orthodox leaders view the Eastern Rite as an attempt to destabilize their churches.

An influential Eastern Rite cleric, Cardinal Ignace Moussa Daoud, was dropped from the papal delegation after objections from Greek Orthodox leaders.

Christodoulos plans to travel to Moscow on Saturday for meetings with Russian Orthodox leaders, who have strongly resisted contacts with the Vatican. John Paul's groundbreaking visit to Athens -- the first by a pope to Greece in nearly 13 centuries -- may open opportunities for another landmark papal trip to Russia.

The pope's message also advanced his effort to begin the millennium with prayers of contrition for wrongs committed by Roman Catholics throughout the ages, including abuses against women and minorities. In March 2000 -- on a similar biblical pilgrimage -- the pope visited Israel's Holocaust memorial to say his church was "deeply saddened" by Christian persecution of Jews.

But the rift with the Orthodox may be one of the hardest to bridge.

Christianity split into the two branches nearly 1,000 years ago in disputes over papal authority. Some Orthodox clerics are highly critical of the Vatican and oppose any attempts at reconciliation.

The ill feelings draw from potent sources: religion, ethnic pride and a perception of historical injustices.

The ecumenical effort, however, would receive a major boost if supported by the Greek Orthodox. But there's a risk it could unleash an internal crisis in the Greek church.

"For something new to come about, something old must die," said Nicholas Constas, an assistant professor of theology at Harvard Divinity School.

Protesters -- from monks to parish priests -- planned rallies during the pope's 24-hour stay. They rang church bells in a symbol of mourning and lowered to half staff Greek flags and those of ancient Byzantium with its two-headed eagle. Priests released black balloons with signs reading "pope go home."

Riot police blocked demonstrators from coming near Athens' Roman Catholic cathedral before the pope's scheduled appearance.

"The Vatican is the house of deception and criminal activity," shouted a Greek Orthodox cleric, Metropolitan Stephanos, through a bullhorn.

But large-scale opposition appeared to fizzle just hours before the pope's arrival. Some former protest leaders appealed for calm, apparently bowing to intense pressure from the government and mainstream church leaders.