New Chinese rules give the country's coastguard the right to detain foreign nationals in the South China Sea for up to 60 days without trial.
Beijing claims sovereignty over most of the sea and dismisses claims from several other countries, including the Philippines. The Chinese coastguard has used water cannons against Filipino boats in the disputed waters on several occasions.
The Philippines is considering a range of measures to protect its fishermen.
The G7 countries criticised China's "militarisation and intimidation activities" in the South China Sea on Friday.
