Anti-China protesters have set on fire several factories at an industrial park in southern Vietnam, amid tensions over the South China Sea.
The park’s management said three factories were set on fire on Tuesday, but other reports put the figure as high as 15.
No casualties have been reported but officials said many arrests were made.
The protests came after China moved a drilling rig into waters claimed by Vietnam earlier this month.
In a daily press briefing, a Chinese Foreign Ministry spokesman said Vietnam was a “provocateur” and that Beijing had expressed concern to Hanoi.
The management of the Vietnam Singapore Industrial Park (VSIP) said that protestors gathered on Monday in Thuan An town, in the southern Binh Duong province.
On Tuesday they moved on to VSIP’s two industrial parks nearby and targeted factories owned or managed by the Chinese and Chinese expatriates.
Not all of the tenants of the three factories were Chinese companies. Some Taiwanese companies had been affected.
Other reports suggested the violence was more widespread, with more factories targeted.
A local official estimated that around 19,000 workers took part in the protest and that at least 15 factories were set on fire, according to local media.
One photo carried by Vietnamese media showed a factory had draped a South Korean flag at its entrance in a bid to stave off attacks.
Another eight were partially damaged, and had shattered windows and smashed front gates. These included buildings belonging to a Taiwan-founded shoe company.
The protest has spooked some foreign companies. Reuters reported that Hong Kong-listed sports shoe maker Yue Yuen, which supplies footwear to Adidas, Nike and other international brands, had suspended production in Vietnam.
Earlier this month, China moved its Haiyang Shiyou 981 oil rig to a spot 120 nautical miles off the coast of Vietnam.
The area is near the Paracel Islands, over which China and Vietnam have contesting claims.
The move sparked bitter protest from the Vietnamese government, which demanded an immediate pull-out.
Last week, several collisions were reported between ships from the two countries as Vietnam sought to block the installation of the rig.
Ships have also been exchanging water cannon fire and dozens of vessels are reported to be in the area.
Protests have been staged in Hanoi and Ho Chi Minh City over the past week. Vietnamese activists marched to the Chinese embassy in Hanoi on Sunday and again on Tuesday.
On Tuesday, the US warned China that its actions were “provocative”.
Beijing claims a U-shaped swathe of the South China Sea that covers areas other South East Asian nations say are their territory.
The issue has been rumbling in recent years amid an increasingly assertive stance from China over its claims.
The Philippines on Wednesday accused China of reclaiming land on a disputed South China Sea reef in order to build a new facility – possibly an airstrip or a military base.
Manila lodged a protest last month after images taken from the air showed China had been moving materials into Johnson Reef in the Spratly Islands, officials said.
Ties between Beijing and Manila have deteriorated severely in recent months because of the territorial row.
Manila is taking Beijing to an international court over the issue. It also recently signed a security deal with the US allowing more troops onto its soil, in a move seen as reflecting the difficult ties with China.