China warned its citizens in Manila to stay off the streets and take precautions during planned anti-Chinese protests on Friday, a sign of mounting tensions during a standoff in the resource-rich South China Sea.

The planned protest of about 1,000 people could heighten tensions between China and the Philippines over what is known as the Scarborough Shoal in English and Huangyan island in Chinese in the southeastern region of the disputed waters.

On Tuesday, China’s embassy in Manila posted an advisory on its website asking its citizens to stay indoors, avoid demonstrations and refrain from confrontations with locals, anticipating a “large-scale” anti-China protest on Friday.

That coincided with a stern warning from Beijing that China had made “every preparation” to counter what it called potential expansion of the conflict by the Philippines, one of Washington’s closest allies in Asia.

Civil society and political groups with links to President Benigno…