Police stand guard ahead of the Olympic torch relay in Nagano, Japan. (Junko Kimura/Getty Images)

Japanese runners bypassed minor protests Saturday and carried the Olympic torch through Nagano's streets, lined by thousands of riot police and closely monitored by helicopters overhead.

Police guards in track suits surrounded the first runner — the manager of Japan's national baseball team — and another 100 uniformed riot police trotted alongside six patrol cars and two motorcycles.

They were backed up by thousands of other police.

Japanese officials said the security was unavoidable, and called for calm. But the high-profile police presence dissipated any festive mood in Nagano, which hosted the 1998 Winter Games.

Despite a heavy police presence, minor scuffling and protests broke out. At one point, a man tried to charge the torch but police quickly pounced.

National broadcaster NHK reported a smoke-emitting tube was thrown at the relay, but without effect.

Marchers yelling "Free Tibet" crowded the streets near the route. And before the start, one person was hurt in a fight between Chinese and pro-Tibetan supporters, and a self-proclaimed monk carrying a knife was arrested.

The starting point — a last-minute substitution after a Buddhist temple pulled out — was closed to the public, as were all rest stops along the way.

The relay, making its 16th international stop, has been disrupted by protests or conducted under extremely heavy security at many sites since it left Greece.

The protests are largely in response to China's crackdown last month on protests in Tibet, which it has governed since the 1950s, and to concerns over human rights issues in China.

The international route ends next week, with stops in South Korea on Sunday, North Korea on Monday and Vietnam on Tuesday.

The flame arrives on Chinese soil on May 2 in Hong Kong, for a long journey around the country before the Aug. 8 start of the games.

Japan has taken severe measures to ensure its 18.6-kilometre relay goes smoothly.

But groups including Amnesty International and Reporters Without Borders planned to protest peacefully throughout the day. About 2,000 Chinese exchange students, meanwhile, swarmed Nagano to show their support.

Coinciding with the start of the relay, which began under a light rain, a prayer vigil was held at the largest Buddhist temple in Nagano, Zenkoji.

The 1,400-year-old temple, which was the showcase of the 1998 Olympics, last week declined to host the start of the relay, citing security concerns and sympathy among monks and worshippers for their religious brethren in Tibet.

After arriving in Nagano by bus early Friday, the flame was spirited away to a hotel and put under heavy security. About 3,000 police have been mobilized.

The problems with the torch relay and reports of foiled terrorist plots in China have raised larger concerns of violence during the Beijing Games, the head of Interpol said Friday.

Ronald Noble told an international security conference that potential attacks could involve efforts to block transportation routes, interfere with competitions, assault athletes or destroy property during the Olympics.

In Vietnam, authorities expelled an American citizen of Vietnamese origin who planned to disrupt the relay there, state media reported.

Vuong Hoang Minh, 34, was put on a flight to the U.S. on Thursday, the Vietnam News Agency said. It said Minh told authorities he planned to snatch the torch.