HO CHI MINH CITY, Vietnam — Vietnam tightened security and detained several people today, witnesses said, ahead of the final international leg of an Olympic torch relay that has been dogged by protests against China's government.

A small group of Chinese citizens waved flags and shouted "Go Beijing! Go!" as workers prepared the stage for the opening ceremony of the parade in downtown Ho Chi Minh City, formerly known as Saigon.

Several police officers were stationed close to the starting point outside the southern city's 19th century opera house.

In the capital Hanoi, police broke up an anti-China demonstration, witnesses said. Several people were detained for unfurling a banner and shouting "Boycott the Beijing Olympics" through a loudspeaker in a market, according to two witnesses. They spoke on condition of anonymity because they were afraid of getting into trouble with authorities.

Authorities refused to comment on the incident in the capital, where scores of riot police blocked the road in front of the Chinese Embassy.

Vietnam has assured its communist ally and giant northern neighbor it will not allow demonstrators to disrupt the parade, though students have threatened to protest China's claim to the disputed Spratly Islands.

The torch arrived in Vietnam late Monday from North Korea, where tens of thousands of citizens were mobilized to celebrate the relay in Pyongyang in the flame's first visit to the authoritarian nation.

Tuesday's parade will start in the early evening outside Ho Chi Minh City's 19th century opera hall. But Vietnamese authorities have given few other details about the relay, including its route, apparently for security reasons.

China and Vietnam fought a border war in 1979, but relations between the two communist countries have improved greatly in recent years.

At many of its 18 stops, the relay has been beset by protests against China's human rights record and a recent crackdown in Tibet following anti-government riots. Large groups of patriotic, flag-waving Chinese have also turned out, in some cases clashing with protesters.

From Vietnam, the flame will travel to the Chinese territories of Hong Kong and Macau before heading to the mainland, including the restive Tibet region and to the top of Mount Everest.