Anti-China protesters have set more than a dozen factories on fire in Vietnam in the biggest eruption of rage against Beijing for decades over the deployment of an oil rig in contested waters.

China expressed "serious concerns" after Vietnamese workers went on the rampage Tuesday, looting goods and attacking offices in a rare outburst of public unrest in the authoritarian communist nation.

Riot police were deployed after violence in the southern province of Binh Duong forced several factories to temporarily suspend operations, including a supplier for Nike and Adidas.

Taiwanese and South Korean plants were affected along with Chinese factories.

"Huge fires have engulfed many of the Taiwanese plants. It would be impossible to estimate the losses. The attacks were totally unexpected," a Taiwanese man who fled the unrest told reporters at an airport in northern Taiwan.

Police said they had detained 500 people for looting and arson, as the authorities struggled to cool tensions that have boiled over since Vietnam's communist rulers -- who usually tightly control dissent -- allowed mass rallies against Beijing at the weekend.

The riots show the "hazards of nationalist fervour unleashed, particularly in repressive institutional environments such as Vietnam," said Professor Jonathan London at City University of Hong Kong.

Nearly 20,000 workers poured onto the streets Tuesday and a hardcore began looting and attacking security guards and factory management before setting fire to at least 15 factories, local authorities said in a statement.

There was a "massive mobilisation" of local forces, with riot police brought in as reinforcements, the Binh Duong People's Committee said.

Videos and images posted on dissident blogs showed thousands of workers -- many waving the Vietnamese flag -- destroying factory gates, smashing windows and damaging offices.

Export-orientated manufacturing is a key pillar of Vietnam's economy, with high-profile firms -- from electronics giants such as South Korea's Samsung to US sportswear companies -- producing goods there.

- Calls for restraint -

China made "solemn representations" and asked Vietnam to take all necessary steps to stop and punish the crimes, foreign ministry spokeswoman Hua Chunying told reporters in Beijing.

A number of Taiwanese, Japanese and South Korean businesses have reportedly temporarily shut their plants and sent workers home, hanging Vietnamese flags outside their business in a bid to deter looters.

"We made the decision to give our people a day off today as the situation is pretty tense in Vietnam right now," said Jerry Shum of Taiwanese footwear manufacturer Yue Yuen, which is a supplier to brands such as Nike and Adidas and employs around 100,000 people in Vietnam.

Taiwan condemned the violence and said it had called on Vietnam to guarantee the safety of its nationals.

"We urge the Vietnamese people to exercise restraint and not to take violent and non-rational actions as this would affect Taiwanese businessmen's willingness to invest," Foreign Minister David Lin said.

Singapore, Vietnam's second-largest foreign investor after Japan, called on Hanoi to take urgent action before "the security situation worsens and investor confidence is undermined".

The riots were the worst anti-China unrest since reunification in 1975, according to Vietnam expert Carl Thayer, a professor at the University of New South Wales in Australia.

There could be an element of "latent economic grievances" surfacing in the attacks on factories, he said.

The authoritarian government will crack down hard on the violence as it is concerned that it could "mushroom into protests against corruption, jailing of bloggers, human rights and religious freedom," Thayer added.

- Old rivalry boils over -

China and Vietnam are locked in long-standing territorial disputes in the South China Sea over the Paracel and Spratly islands, which both claim.

There have been repeated skirmishes near the controversial oil drilling rig in recent days involving vessels from the two countries, with collisions and the use of water cannon.

Beijing said Vietnamese ships rammed its vessels 169 times on Tuesday. Hanoi has also accused Chinese ships of ramming its vessels.

Southeast Asian leaders voiced "serious concern" over the worsening sea tensions at a summit on Sunday in Myanmar, after Vietnam and the Philippines led a successful push to put Beijing's territorial assertions high on the agenda.

The Philippines warned on Wednesday that China may be building an airstrip on a reef in the South China Sea as the Asian superpower asserts its claim to most of the strategic area.