VATICAN CITY (Reuters) - Pope Benedict prayed for peace on Friday as he led the world's Roman Catholics into Christmas at a mass marked by heightened security following parcel bomb attacks at two embassies in the Italian capital.

"At this hour, full of thankfulness, we join in the singing of all the centuries, singing that unites heaven and earth, angels and men," Benedict said in his sermon at a traditional Christmas Eve Mass in St Peter's Basilica attended by some 10,000 people.

"Grant that we many join with you in love more and more and thus become people of peace," he said, wearing resplendent gold and white vestments.

Celebrating his sixth Christmas as pope, the 83-year-old Benedict earlier lit a candle at his window overlooking St Peter's Square for the official opening of the Vatican's nativity scene but a torrential downpour kept attendance low.

Police were on heightened security in the Vatican and in Rome a day after parcel bombs at the Swiss and Chilean embassies in Rome. Anarchists claimed responsibility for the attacks, which injured one person at each embassy.

"His guardian angel will protect him and each one of us will be protected," said tourist Gayle Savino, from New York, as she entered the basilica for the mass. "It's just a blessing to be here on such a wonderful night on Christ's birthday."

VATICAN SECURITY BREACHES

Vatican guards were also more vigilant following security breaches for two consecutive years at Christmas Eve masses by the same woman, Susanna Maiolo.

Last year the woman, who has had a history of mental problems, jumped over a barricade as the pope walked up the basilica's main aisle and managed to pull him to the floor. The year before, she was stopped before she could reach him.

Large televisions screen were placed outside in St Peter's Square but the rain kept many people away.

On Christmas Day the pope celebrates a mass and then delivers his twice-yearly "Urbi et Orbi" message and blessing from the central balcony of St Peter's Basilica and read Christmas greetings in more than 60 languages.

On Saturday, he will play host to some 350 homeless people from Rome shelters for a meal in the Vatican's audience hall.

The group claiming responsibility for Thursday's parcel bomb attacks in Rome, the Informal Anarchist Federation (FAI), is well known to Italian police and was described in an intelligence report to parliament last year as "the main national terrorist threat of an anarchist-insurrectionist type".

It gained notoriety in 2003 with its so-called "Santa Claus campaign" against EU institutions which included a parcel bomb sent just before Christmas to Romano Prodi, a former prime minister who at the time was head of the European Commission.

A climate of tension has pervaded Italy in recent weeks amid austerity measures that have hit public services during the financial crisis and student protests last week that spiralled into some of the worst violence in Rome in years.