The activist, who holds a UK refugee travel document, told AFP that he had been denied entry when he tried to pass immigration after his plane landed at Singapore's Changi Airport from San Francisco on Saturday.

He said he was sent back on Sunday on a plane to San Francisco.

"Law's entry into and presence in the country would not be in Singapore's national interests," the Ministry of Home Affairs (MHA) told AFP in a statement.

"A visa holder is still subject to further checks at point of entry into the country. That is what happened with Nathan Law."

Law had fled from Hong Kong in 2020, and the police there issued a warrant of arrest against him for offences under their National Security Law, the MHA said.

Upon arrival in Singapore, Law was "referred for questioning, and immigration and security assessment" at the airport, according to the MHA.

"Law was then refused entry. He was placed on the earliest flight back to San Francisco on 28 September 2025," the ministry added.

Law said he was granted a Singapore visa to attend a "closed-door, invitation-only event" and expected to be allowed in.

"I received the visa around three weeks prior to departure... I submitted all information, including the invitation to the event in the application," he told AFP.

"I attempted to go through immigration, but I was detained, despite having a valid visa to enter the country.

"I was not told why I was detained," he said, adding he spent "roughly 14 hours" in Singapore.

He said he thought Singapore's decision to deny him entry was "political, although I am unsure whether external forces, such as the PRC, are involved, directly or indirectly", referring to China by the initials of its formal name.

Asked about the case on Monday, China's foreign ministry said it was not familiar with the situation but that countries had the right to manage their own immigration processes.

"The person you mentioned is an anti-China, destabilizing force in Hong Kong who is lawfully wanted by the Hong Kong police," spokesman Guo Jiakun added.

Law was one of the student leaders of the 2014 Umbrella Movement in Hong Kong, the sometimes violent pro-democracy protests in the financial hub which Chinese officials saw as a challenge to their rule.

He has lived in exile in Britain and the United States since fleeing Hong Kong after Beijing imposed a national security law on the territory.

Singapore has refused entry to foreign activists in the past, saying the country cannot be used as a platform for political activities.