SAN FRANCISCO — At least one runner bowed out, others were on edge and police began to deploy a tight, closely guarded security plan Tuesday as the city braced for conflict today around the only North American stop for the Olympic torch.

Police began checking the bags of bicyclists and pedestrians crossing the Golden Gate Bridge and Mayor Gavin Newsom sought to ease the fears of torchbearers at a meeting inside a Union Square hotel.

Runners said they were told a moving phalanx of police would shield them from the kind of violence and flame-grabbing that forced an early end to Monday's run in Paris, and could lead Olympic officials to abort the flame's journey to Beijing.

International Olympic Committee president Jacques Rogge said officials will discuss Friday whether to call a halt to an 85,000 mile journey that began in Greece and has been beset by violent protests over the Chinese government's crackdown on Tibet and other human rights charges.

"We recognize the right for people to protest and express their views, but it should be nonviolent," Rogge told the Associated Press.

"We are very sad for all the athletes and the people who expected so much from the run and have been spoiled of their joy."

The flame arrived in San Francisco by airplane early Tuesday morning and was sped off to a secret location.

A San Francisco police spokesman said every city officer will be on duty Wednesday, and the plan includes state and federal security forces.

Newsom also said the 6-mile waterfront route could be altered, and several side streets were marked off Tuesday with police notices.

After three protesters scaled the famed Golden Gate Bridge on Monday to raise the Tibetan flag and a pair of banners, police swarmed around United Nations Plaza on Tuesday during an evening rally for a free Tibet.

A spokesman for the city said one runner had backed out of the San Francisco relay, but other torchbearers said officials told them two had decided not to run.

Runners said they were given few specifics and would not know their place on the torch relay until Wednesday morning. The violence overseas drew concern, and in some cases fear among some of the original 80 torchbearers.

"I've walked into this maelstrom I didn't anticipate," said Dean Karnazes, 45, an ultramarathon runner from San Francisco who months ago submitted an essay that won him a spot in the relay.

Karnazes said protesters contacted him, and a few stood outside his house, hoping he would take up their cause.

He said Tuesday's meeting room was "full of a lot of concerned folks." One was Jasmine Nachtigall, a 17-year-old half-Chinese student from San Mateo who called the last week "very frightening."

"I've been going on pretty long runs lately, trying to translate this fear into a sort of courage," said Nachtigall, a volunteer for Amnesty International.

She said she has taken to singing "Stop in the Name of Love" in her head to cope with the fear, but that word of the security measures has eased her mind.

"We have to remember these protesters are protesting the government," she said.

Others remained excited, and confident in the security measures.

"It's a dream come true," said Morgan Montoya, 16, of Benicia, who will run in honor of her brother, Max, who will undergo daylong surgery Wednesday to correct life-threatening spinal deformity.

"I have no doubts," said Neshat Rezai, a Berkeley dentist who will also run. "I think San Francisco is the right city to have this. It shows the city all sides."

Early this month, the San Francisco Board of Supervisors passed a resolution criticizing China's human rights record and pledging to greet the torch with "alarm and protest." House Speaker Nancy Pelosi, of San Francisco, blamed the Chinese government for turning the torch relay into a political event.

Pelosi also called on protesters in San Francisco to remain peaceful.

Along the Embarcadero on Tuesday, strollers were left to wonder whether violence would erupt along the parade route or elsewhere, and how the city's response might influence its reputation.

In the nation's most accommodating big city for protests, several pledged to steer clear of the waterfront.

"Free Tibet, I'm all for that, but don't take it out on a person with a torch. I don't want to see people chasing little girls," said Richard Canty of Moraga, sitting at the bar of Pier 23.

"If San Francisco screws it up, it's going to be bad," added bar mate Charles Hall. "This will be a pretty good test."

Associated Press contributed to this report.