China says the North Koreans are economic refugees. It has been criticised by human rights groups for repatriating them, where they face prison terms under life-threatening conditions in brutal camps.
South Korea police said they arrested two Chinese on suspicion of throwing rocks at anti-China protesters, one North Korean defector who tried to disrupt the relay and two other defectors who poured paint thinner on themselves in an apparent attempt to set themselves on fire.
There are several thousand Chinese students in South Korea and they were bussed in from all parts of the country, supplied with Chinese flags, T-shirts, banners and stickers.
The pro-Beijing rallies in the South Korean capital left many Seoul residents angered.
"It's OK to cheer for their Olympic torch, but this is too much," said Min Chae-woo.
Protests, though, will not be on the carefully planned agenda at the flame's next stop, the North Korean capital Pyongyang.
Human rights groups say North Korea's authoritarian leaders crush any attempt at dissent. A protest of any sort is certain to lead to at least a long sentence in a political prison, or even execution.
The torch is expected to arrive with its attendants by airplane in Pyongyang at around midnight (1500 GMT). It then goes to Vietnam and Hong Kong.
Vietnamese security forces alone will guard the torch in Ho Chi Minh City, officials said on Sunday, as political activists called for demonstrations over an island dispute with China.
In Hong Kong, three human rights activists who planned to protest against rights violations in China during the torch relay in the city were refused entry to the city, a newspaper reported on Sunday.
"TOO NOISY"
The isolated North, which rarely holds international events, has promised China it will stage an "amazing" relay on Monday.
When North Korea hosts an honoured state visitor, it sends hundreds of thousands of its citizens into the streets of Pyongyang. Dressed in their finest clothes, they wave bouquets of pink and purple plastic flowers and cheer on cue when the guest passes by.
In freewheeling Seoul, security was elaborately planned, involving about 8,000 police officers, but some South Koreans were unimpressed.
"This is too noisy," said an elderly South Korean woman named Park who walked past the start of the relay.
"I wish that the torch never came here and all of this would go away."
