FACTBOX: The Olympic torch in history

(Reuters) - The Beijing Olympic torch arrives in San Francisco for its only U.S stop on Wednesday, where more of the protests that have dogged the relay's initial international leg are expected.

Here are some facts about the Olympic torch and past and present controversies.

HISTORY OF THE FLAME:

* Flame races were run in ancient Athens to honor deities including Prometheus, who, legend has it, stole fire from the gods and brought wisdom and knowledge to humankind.

* A fire was kept burning at ancient Olympics to honor Zeus, the supreme ruler in the Greek pantheon of gods.

* Fire first reappeared at the modern Olympics in Amsterdam in 1928, when a flame was built into an Olympic stadium tower. It was seen as a symbolic link between old and new games.

TORCH-LIGHTING CEREMONY:

* The torch is ignited several months before the games start at the ruins of the Temple of Hera, in Olympia, southern Greece, site of the ancient Olympics. Actresses dressed as priestesses use the sun and a parabolic (curved) mirror to light the flame.

* Carried on foot to Athens in an urn, it is delivered to host city officials at the Greek capital's Panathinaiko Stadium. Shielded in a security lamp, it then travels on to the host city on foot, or via boat, airplane, bicycle, car or train.

* The first modern Olympic torch relay was introduced for the 1936 Berlin Games and was the brainchild of Nazi-sponsored Olympic official Carl Diem, who choreographed the 1936 Olympics.

-- Despite its latter-day peaceful symbolism, the first torch was made of sterling silver from Krupps metal works where Adolf Hitler was stockpiling munitions ahead of World War Two.

-- More than 3,300 torchbearers carried the torch 1,980 miles from Olympia to Berlin.

* The Athens' 2004 Summer Olympics saw the first global torch relay through 34 cities in 27 countries.

RECENT TORCH CONTROVERSIES:

* 2000 - The torch-lighting ceremony for the Sydney Olympics comes under fire for switching the first handover from a Greek-Australian girl to the 11-year old daughter of an International Olympic Committee member.

* 2005 - In December 2005 around 100 anti-globalization protesters, opposed to Coca Cola, a sponsor of the 2006 Turin Winter Olympics, and the construction of the Turin-Lyons high-speed railway, stop the torch in Genoa, Italy. The torch was extinguished as a precaution.