Nathan Law said he was detained at the Singapore airport on Saturday night and told four hours later that his entry had been denied.
He was going to attend a closed-door, invitation-only event, he said in a statement, without elaborating.
Mr Law is one of a number of activists who have fled Hong Kong in recent years after the adoption of a national security law that has thrown others in prison.
He said he had received a visa from Singapore to attend the event and departed on a flight from San Francisco. No reason was given for the denial, he said, and he boarded a plane back to San Francisco on Sunday after about 14 hours in Singapore.
“I think the decision to deny my entry was political, although I am unsure whether external forces, such as the PRC (People’s Republic of China), are involved, directly or indirectly,” his statement said.
Mr Law declined comment further.
A Singapore government statement confirmed that he had been refused entry, saying a visa holder was subject to further checks at the border.
It noted that Hong Kong police had issued a warrant for Mr Law’s arrest under the city’s national security law.
“Law’s entry into and presence in the country would not be in Singapore’s national interests,” the statement from the home affairs ministry said.
Mr Law is accused of colluding with foreign parties and inciting secession for allegedly calling for sanctions and Hong Kong’s separation from China in meetings with foreign officials and in open letters, petitions, social media posts and media interviews.
Hong Kong is a Chinese territory but has its own laws and regulations. The central government in Beijing cracked down after massive anti-government protests in 2019, imposing a national security law on Hong Kong the following year.
Mr Law rose to prominence as a student leader of the pro-democracy Umbrella Revolution in 2014 along with Joshua Wong, who is imprisoned in Hong Kong.
In 2023, Hong Kong police offered rewards of one million Hong Kong dollars (£95,000) for information leading to the arrest of Mr Law and other self-exiled activists for national security law violations.
Mr Law was elected to the legislature in 2016 but disqualified after he raised his tone while swearing allegiance to China during the oath of office, making it sound like a question.
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