By Christian Fraser BBC News, Rome

The guards celebrated with a mass in the Sistine Chapel

They are known as the "Defenders of the Church" and this weekend the Vatican Swiss Guard celebrated 500 years of service.

In that time they have protected 42 successive popes, although more recently the Vatican has been guarded by Italian security forces and plainclothes police. But personal safety of the pope is still the guards' full responsibility.

The guards first arrived in Rome on 22 January 1506. At that time, Helvetian soldiers were employed as mercenaries, renowned for their courage and their loyalty.

There were many famous battles, but their most notable hour came in 1527, during the sack of Rome. Almost the entire guard was massacred by troops of the Holy Roman emperor Charles V on the steps of St Peter's Basilica.

Of 189 guards on duty only 42 survived, but their bravery ensured that Clement VII escaped to safety, down the "passeto", a secret corridor which still links the Vatican to Castel Sant' Angelo.

Swiss elite

Popular myth says the traditional yellow, blue and red uniforms the guards wear - the colours of the Medici - were designed by Michelangelo. But in truth they are only 100 years old and were designed in 1905 by a Swiss Guard Commander.

The weapons they carry were first introduced by the early Swiss armies in the 14th Century. It is an axe, known as the halberd, mounted on a long shaft with a spike at the end. When travelling with the pope, the uniforms and the halberd are left behind.

The Halberdiers protect the main entrances to the papal palace and the papal apartments. During the conclave they were also on guard at the doors of the Sistine Chapel while Benedict XVI was being selected as the new pope.

Currently, there are 110 guards and their numbers are limited by law.

The guard says it gets three times as many applicants as there are openings ever year. Applicants must be Swiss Catholics, have finished basic training in the Swiss army and be at least 5ft 8in (1.73m) tall. Minimum enrolment is two years.

Murder

The guards' worst moment came in 1998 when the new commander, Alois Estermann, and his wife were found dead in their apartment.

The Swiss guards were at Pope John Paul II's side until the end

The man who killed the couple, and later himself, was a disgruntled young corporal annoyed at being overlooked for a medal.

In 1981, as a young captain, Estermann had tried to protect John Paul from a Turkish would-be assassin. When the shots rang out he jumped into the pope's vehicle to shield his body but the pope had already been seriously wounded.

Veterans of the guard are taking part in the 500th anniversary celebrations. Official ceremonies began in September with a mass and rally of veterans in Lucerne, in central Switzerland.

But the main event will be a march to Rome, from Bellinzona in southern Switzerland, by around 80 veterans, to recall the march of the original 200 in 1505.