A woman vaulted a barrier inside St Peter's Basilica and knocked Pope Benedict XVI to the floor as he arrived to celebrate Mass marking the beginning of Christmas. The 82-year-old pontiff was uninjured and was able to deliver his Christmas Eve homily. Vatican officials said the woman appeared to be mentally unstable and was arrested by Vatican police. Earlier, in Bethlehem, thousands of pilgrims from around the world descended on the traditional birthplace of Jesus, for the most upbeat Christmas celebrations the Palestinian town has seen in years.