Italy has imposed unprecedented security measures for the Easter weekend to thwart any terrorist attack, particularly against the Vatican or Pope John Paul II.
Details of the measures, announced piecemeal by officials over the past week, were published in newspapers on Friday as millions of Italians and tourists began moving around the country for the long holiday weekend.
They include a ban on small aircraft over Rome, with jet fighters and helicopters ready to take to the air within minutes to intercept intruders.
Security has always been intensified over the Easter holidays in Italy because of crowds.
Officials said this year's precautions were on a new scale following the March 11 train blasts that killed 191 people in Madrid and with the deteriorating situation in Iraq, where Italy has some 3,000 troops.
Pope John Paul, who was shot in an assassination attempt in St Peter's Square in 1981, will be making five appearances in Rome and the Vatican, including an outdoor Easter mass that usually attracts hundreds of thousands of people.
Police have sealed manhole covers near the square and traffic will be diverted during the night to thwart possible suicide car bombers.
Alfredo Mantovano, an undersecretary at the interior ministry, told Italian radio on Friday morning that some 19,000 police and 4,000 military would be working over the weekend.
"What happened in Madrid on March 11 has increased concern," Mantovano said, adding however that both "excessive alarmism and banal reassurances should be avoided".
"We have to see to it that terrorism does not reach its objective of spreading panic," he said, adding that on Easter Sunday he would be in St Peter's Square to hear the pope.
Police and troops will be protecting more than 13,000 sites deemed to be potential targets, including such tourist meccas outside Rome as the Leaning Tower of Pisa, St Marks's Square in Venice, Milan's cathedral and Florence's Uffizi museums.
Easter falls on April 11 and some newspapers have made a link to the September 11, 2001 attacks against the United States and the March 11 bombs in Madrid a month ago.
On Thursday, a top anti-terrorism investigator said militant Islamic cells scattered across Italy, many of them so far used to support attacks abroad, could turn their sights on targets inside the country.