"You can't invite thousands of people and then bottleneck the entrance and exit to the square," said Gianbattista Di Rese, an Italian among the distressed. "Imagine if someone had had a bomb. There could have been hundreds of dead." He got into the square but was stymied trying to get out.
Tourists must go through metal detectors before entering St. Peter's Basilica, but there is no such security measure to stroll the square.
An Associated Press reporter saw many people give up. Some started to panic and yell at police to do something to ease the bottleneck.
Those who arrived hours before the pope appeared could enter the square with ease for a chance to join in a show of support for him.
"We wanted to wish him well," said Amy Champion, a tourist from Wales. "It takes a lot of guts to take the job and even more guts ... to quit."
But some were dismayed that Benedict broke with the centuries-old tradition that popes serve till their last breath.
A youth group Militia Christi (Latin for Christ's Militia) held a hand-painted banner asking the pope to stay. "We are asking him to change his mind. He is the good of the church," said youth GiovanBattista Varricchio.
No decision has been announced on a date for the conclave to elect Benedict's successor, but the Vatican has suggested that it might start sooner than March 15, the earliest date possible under current rules, which require a 15-20 day waiting period after the papacy becomes vacant.
This has set off a debate whether such a change could be justified and whether it might benefit Rome-based cardinals who because of their positions at the church's headquarters can count on their acquaintance with cardinals around the world.
"Church law should not be changed on a whim," said the Rev. Thomas Reese, an American Vatican expert. He said changing law "would be disruptive."
Daniela Petroff contributed reporting.
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