New maritime regulations take effect in China on Saturday, giving the coast guard new powers to detain foreign nationals deemed to have illegally trespassed into the country's waters.
Suspects can be detained for 30 days, or up to 60 days in more complex cases.
Authorities created the regulations based on a law introduced in 2021 allowing the coast guard to use weapons.
The Philippines views the latest move as an attempt by Beijing to consolidate its control over the South China Sea, where the two countries have territorial disputes.
Last month, Philippine President Ferdinand Marcos Jr. said China's plan is "completely unacceptable."
Officials with the US Indo-Pacific Command have expressed concern about a possible escalation in tensions.
The Japanese government is poised to closely watch China's maritime activities so that the legitimate interests of other countries won't be harmed.
Masuda Masayuki, an expert at the National Institute for Defense Studies, says China could soon step up its law enforcement activities in the South and East China seas, including the Senkaku Islands in Okinawa Prefecture.
Masuda suggests that China may be expanding so-called grey zone tactics without resorting to open combat.