President Bush has paid last respects to Pope John Paul in Saint Peter's Basilica after arriving in Rome Wednesday evening.
The president was accompanied by his wife Laura, former Presidents Bill Clinton, George H.W. Bush and Secretary of State Condoleezza Rice.
Some 200 political and religious leaders are due in Rome for Friday's funeral.
Officials say to protect the dignitaries Rome is erecting a defense shield that will include a no-fly zone and anti-craft missiles.
Thousands of police reinforcements are arriving from across Italy, with authorities expecting up to four million visitors.
Authorities are also urging pilgrims to stay away from central Rome, saying that due to huge crowds any newcomers would have no chance of seeing Pope John Paul's funeral.
The head of crowd control for the funeral, Guido Bertolaso is asking pilgrims to go to a special camp on the outskirts of the city.
With the expected wait up to 24 hours, police cut off mourners Wednesday evening from joining the end of the line in front of Saint Peter's Basilica to allow authorities to prepare for the funeral.
Meanwhile, Roman Catholic cardinals, who will choose a new pope, have set April 18 as the date for the start of closed-door deliberations.
Some information for this report provided by AP, AFP and Reuters.