VATICAN CITY — After electronic highway signs and cell phone text messages failed to stanch the flow of pilgrims, police stepped in Wednesday to turn back mourners hoping to join the 24-hour line to view the body of Pope John Paul II, on a day that brought almost 1 million people to the Vatican.

The crowd-control problems developed hours after the College of Cardinals set April 18 as the start of its conclave in the Sistine Chapel to choose a successor to John Paul II, a papal election with new rules and new technological challenges.

Using a special entrance for VIPs, President Bush viewed the body with his wife, Laura, along with his father, former President George H.W. Bush, former President Bill Clinton and Secretary of State Condoleezza Rice, shortly after the U.S. delegation reached Rome. They knelt in a pew in front of the remains, bowing their heads in prayer, joining a million pilgrims who had filed solemnly through St. Peter's Basilica.

Seeking to clear the basilica by tonight so the Vatican could prepare for John Paul's funeral the following day, police announced they were closing the line at 10 p.m. Wednesday. Text messages were sent over Italian cellular phone lines. Those at the back would wait 24 hours before entering the basilica.

"We're just hoping the order can be reversed," said Federica Bruni, a 20-year-old student who came from northern Italy and was one of the first to be told to go away Wednesday night.

"It's possible there are 1 million people out there," said Luca Spoletini of the Civil Defense Department. "They are all concentrated outside St. Peter's. . . . We are all working to ensure maximum tranquility."

At the United Nations, General Assembly members stood in silent tribute to the pope on Wednesday and diplomats offered condolences to his native Poland and the Holy See, which has observer status at the U.N.

The Vatican is a keeper of secrets without parallel, but there were questions Wednesday about whether the deliberations in the conclave — and the name of the new pope — could be kept within the frescoed walls in an era of cell phones and now that the cardinals will be allowed to roam freely around the Vatican.

"They've assured us there are ways to block all communications and conversations," Chicago Cardinal Francis George said. "They're taking precautions to prevent outside interference. . . . No cell phones, no laptops, nothing."

The severest of punishments — including excommunication and "grave penalties" meted out by the pope himself — await anyone who breaks the sacred oaths of secrecy.