Singapore police are investigating an Indian national for allegedly being involved in a public protest against Prime Minister Narendra Modi's controversial citizenship law.

Unauthorised public assemblies and protests over political situations in other countries are banned in Singapore.

Hundreds of thousands of people have taken to Indian streets to protest against the citizenship law enacted by Modi's Hindu nationalist government that provides non-Muslim minorities from Afghanistan, Bangladesh and Pakistan who moved there before 2015 a pathway to Indian citizenship.

CAA protests
Local residents offer prayers on a road during a protest against a new citizenship law, in New Delhi.Reuters

Singapore police said following a report on December 24 they were investigating a 32-year-old male Indian national for participating in "a public assembly without a police permit" at the Marina Bay waterfront financial and tourist district.

"He allegedly carried out the activity in Marina Bay, to show his opposition to India's Citizenship Amendment Bill," police said in a statement late on Wednesday.

The statement did not give any more details of the assembly.

Local media reported the man posted a picture of himself on social media with a placard "to express his unhappiness".

The police said organising or participating in a public assembly without a police permit in Singapore is illegal and that it would not grant any permit for assemblies that advocate political causes of other countries.