Thousands of Christian pilgrims gather in Jerusalem church for ancient holy fire ceremony

Christian pilgrims hold candles as they gather at the church of the Holy Sepulcher, traditionally believed to be the burial site of Jesus Christ, during the ceremony of the Holy Fire in Jerusalem's Old City, Saturday, April 11, 2015. Thousands of Christians have gathered in Jerusalem for the ancient fire ceremony that celebrates Jesus' resurrection. (AP Photo/Dan Balilty) (The Associated Press)

A Christian pilgrim holds up a candle as others gather at the church of the Holy Sepulcher, traditionally believed to be the burial site of Jesus Christ, during the ceremony of the Holy Fire in Jerusalem's Old City, Saturday, April 11, 2015. Thousands of Christians have gathered in Jerusalem for the ancient fire ceremony that celebrates Jesus' resurrection. (AP Photo/Dan Balilty) (The Associated Press)

Christian pilgrims hold candles at the church of the Holy Sepulcher, traditionally believed to be the burial site of Jesus Christ, during the ceremony of the Holy Fire in Jerusalem's Old City, Saturday, April 11, 2015. Thousands of Christians have gathered in Jerusalem for the ancient fire ceremony that celebrates Jesus' resurrection. (AP Photo/Dan Balilty) (The Associated Press)

Thousands of Christians have gathered in Jerusalem for an ancient fire ceremony that celebrates Jesus' resurrection.

In a ritual dating back at least 1,200 years, they crowded Saturday into the Church of the Holy Sepulcher, where Christian tradition holds that Jesus was crucified, buried and resurrected.

During the annual ceremony, top Eastern Orthodox clerics enter the Edicule, the small chamber marking the site of Jesus' tomb.

They then emerge to reveal candles said to be miraculously lit with "holy fire" as a message to the faithful from heaven. The details of the flame's source are a closely guarded secret.

Roman Catholics and Protestants marked Easter last Sunday, in accordance with the Gregorian calendar. Eastern Orthodox churches celebrate Easter this week using the older Julian calendar.