Supporters and opponents of China have clashed in Seoul as the Olympic flame passed through the South Korean capital before going to the rival North.

One man who said he was a North Korean defector tried to set himself alight in protest during the torch relay in Seoul, reports said.

Thousands of Chinese shouting support for the Beijing Games turned a central Seoul plaza into a sea of red flags, mostly drowning out anti-China activists who they greatly outnumbered.

Anti-China activists failed to block the torch but were involved in stone-throwing clashes with Beijing supporters along the 24-kilometre relay route.

Some 300 people at the relay start in Olympic Park protested China's forced repatriation of North Korean refugees.

They were dwarfed by more than 6,000 pro-China supporters, according to police estimates.

Riot police broke up a brief clash at the park between the two groups, with some Chinese throwing water bottles, stones, chunks of wood and cans.

Witnesses said Chinese students beat up a small group of protesters and one threw a stone at a newspaper photographer, hitting him in the head.

Police deployed 8,300 officers backed by two helicopters. They tightly guarded subway stations and the relay route, including the Han River bridges that demonstrators had vowed to block.

Police said they arrested four people -- a Chinese student for hurling a stone at protesters and three demonstrators trying to disrupt the relay. Two policemen were injured.

Among the protesters were North Korean refugees.

NKorean tries to set himself on fire: report

A middle-aged man claiming to have fled North Korea tried to set himself on fire in protest at the event, according to a witness quoted by Yonhap news agency. The man poured what appeared to be flammable liquid over himself near the torch bearer but was stopped by police, the agency said.

"China, stop killing North Korean refugees," read one banner.

China sends back all North Koreans it catches, saying they are economic migrants -- a policy strongly criticised by rights groups. Refugees face severe punishment, or even reportedly a death sentence in some cases, on their return.

Activists say China has been stepping up repatriations ahead of the Olympics which start August 8, and Human Rights Watch has said Seoul should urge Beijing to change its policy on North Korean refugees.

"China, which does not respect human rights, is not entitled to host the Olympics," Reverend Soh Kyung-Suk, co-chairman of Christians for Social Responsibility, told AFP. "It is a shame for South Korea to tolerate the Olympic torch relay for such a country."

But the Chinese students were not inclined to let anyone spoil modern China's coming-out party.

"The Olympics should be successful and will be successful," said a 26-year-old e-commerce student who identified himself only as Wang and carried a large Chinese flag.

Torch welcomed in North Korea

The torch was expected to receive a trouble-free welcome in North Korea, a close ally of China that has strongly criticised a series of pro-Tibet protests that have dogged other global legs of the relay, notably in Paris and London.

Unauthorised demonstrations are strictly banned in the hardline communist state.

A chartered plane carrying the flame landed at Sunan International Airport in Pyongyang early Monday, China's Xinhua news agency reported.

Nearly 1,000 Koreans and Chinese students welcomed the arrival, cheering and holding banners, some reading "warmly welcome Beijing Olympic Flame arriving in Pyongyang," Xinhua said.

Eighty people, including athletes who have competed in international events, will carry the torch, according to official media reports.

Organisers say hundreds of thousands of people will line the 20-kilometre relay route from the Tower of the Juche Idea to Kim Il-Sung Stadium for the event.

A special Olympic flame also arrived at Mount Everest base camp, an attempt by Beijing Games organisers to bring the symbol to the summit of the world's highest mountain, Xinhua said.