BINH DUONG PROVINCE, Vietnam — Dozens of foreign-owned factories near Ho Chi Minh City lay in charred ruins early Thursday after thousands of Vietnamese workers rampaged over China’s latest efforts to control the South China Sea, this time off Vietnam’s coast.

The riots marked a rare outpouring of popular outrage over China’s increasingly insistent claims to strategically important, resource-rich seas. But in their rage, the Vietnamese workers appeared to misdirect their anger, attacking businesses from countries that took the risk of investing in their nation.

The explosion of violence reflected growing animosity in the region as China works to solidify its claims over vast parts of two seas that other nations have long considered their own.

On Wednesday, the conflict played out not only in Vietnam, but also in the Philippines, which said it lodged a formal protest with China over signs that it is reclaiming land at a contested coral reef. But so far, neither the Philippines nor Vietnam has gotten much backing among other Southeast Asian nations, some of which count on China for investment and aid.