The U.S. leg of the Olympic torch relay took place Wednesday in San Francisco amid heavy security, as police SWAT teams prevented a repeat of the chaotic demonstrations that have followed the flame's journey to Beijing.

As the event began Wednesday, San Francisco authorities cut the original relay route nearly in half and changed it, to prevent human rights demonstrations like those that occurred as the torch traveled through London and Paris recently.

At one point after the event started, the first torch runner disappeared into a warehouse with the torch and it was not clear what was happening. The torch later reappeared on a San Francisco street.

San Francisco is the flame's only North American stop and home to one of the largest Chinese-American communities in the United States. The city has drawn thousands of demonstrators gathered to praise and condemn China during the flame's trek to Beijing.

On Tuesday, several hundred activists marched in the city to condemn China's recent crackdown on anti-government protests in Tibet.

On Monday, three protesters scaled the cables of the Golden Gate Bridge to hang pro-Tibet banners. China has condemned the Olympic torch protests as "pitiable and insignificant."

The torch arrived in San Francisco Tuesday and was kept at a secret location until the start of Wednesday's relay. One runner who had planned to carry the torch dropped out. Pro-Tibet activists and human rights groups say they encourage their supporters to protest peacefully without disrupting the relay.

Separately, British Prime Minister Gordon Brown's office said he will not attend the opening ceremony of the Beijing games and German Chancellor Angela Merkel says she has no plans to attend.

President Bush has come under pressure to skip the ceremony to protest Beijing's crackdown in Tibet. On Wednesday, White House spokeswoman Dana Perino said she does not have the president's schedule for that time. She said Tuesday that there was no change in Mr. Bush's plan to attend the Olympics.

Some information for this report was provided by AFP, AP, Bloomberg  and Reuters.