The Turin Shroud has gone back on public display in the Italian city's cathedral, after a break of five years.
The 4.4-metre-long (14-ft) cloth is on show until 24 June.
Viewings are free but must be booked. One million people have already signed up.
Some devotees believe the shroud is the burial cloth of Jesus Christ. Sceptics point to carbon dating that suggests it is a medieval forgery from the 1300s.
Scientists have not agreed on how the image of a man was imprinted on the cloth.
Pope Francis will be among those to view the shroud, during a visit to the northern Italian city on 21 June.
The last public viewings were in 2010, when 2.5 million people came to see it.
The Church has not officially claimed that Christ's body was wrapped in the shroud, preferring instead to focus on what it means to those who see it.
"Whether you believe or don't believe, there is no doubt it is something special," an Italian man who was among the first visitors on Sunday told the AFP news agency.
Earlier this week, Archbishop of Turin Cesare Nosiglia said: "What counts the most is that this shroud... reflects in a clear and precise manner how the gospels describe the passion and death of Jesus.
"It is not a profession of faith because it is not an object of faith, nor of devotion, but it can help faith."
Archbishop Nosiglia said that many of those coming to see the shroud would be returnees.
"That means there is a fundamental need in people's hearts to renew this incredible experience that they had the first time they saw it," he said.
"Even non-believers will come. It's an occasion that brings everybody together."
The shroud is displayed in a climate-controlled case for 12 hours a day in Turin's Cathedral of St John the Baptist.