The Pope has celebrated the traditional Christmas Mass, despite being knocked down by a woman at the start of the service in St Peter's Basilica.

The woman - who is said to be mentally unstable - jumped the security barriers as Benedict XVI entered the basilica, causing the pontiff to fall.

The Pope appeared to be unhurt and recovered to lead the mass.

The midnight service started two hours early because officials do not want the 82-year-old pontiff to get tired.

Thousands of pilgrims have also gathered in Bethlehem to celebrate Christmas mass.

Woman arrested

The security incident involving the woman at St Peter's Basilica lasted only a few seconds.

The woman - who was reportedly wearing a red jumper - tried to jump the barrier and the Pope was thrown off balance and fell, prompting shocked gasps among the crowd.

Security officials rushed down the main aisle to detain the woman.

The Pope had to be helped up by the master of ceremonies at his side, the BBC's David Willey in Rome says.

However, he quickly recovered his poise and passed into the basilica preceded by senior Vatican cardinals, our correspondent says.

Benedict then delivered the Christmas homily in a firm voice.

The woman who pushed the Pope also knocked down Cardinal Roger Etchegaray, who was taken to hospital for a check-up, Vatican spokesman Ciro Bedettini said.

The woman - whose name was not immediately known - was later arrested by police.

During last year's Mass, a woman also jumped the barriers and got close to the Pope but was quickly restrained by security officials.

It was not immediately known if it was the same woman who pushed Benedict on Thursday.

The Vatican said earlier that the decision to move the midnight Mass to 2200 (2100 GMT) was "to make Christmas a little less tiring for the Pope, who has many engagements".

The pontiff earlier appeared briefly at nightfall at the window of his studio to light a candle in a sign of peace.