A pro-democracy campaigner who fled Hong Kong has been denied entry to Singapore despite being granted a visa, with the city-state saying his presence "would not be in its national interests".

Nathan Law, who lives in exile in the UK, said he arrived in Singapore on Saturday to attend a "closed-door, invitation-only" conference but was detained at the border for hours before he was deported.

"I was not asked questions and they did not give reason for the denial," he told the BBC.

Mr Law, who also previously served as a local legislator in Hong Kong, is one of eight exiled activists who is wanted by the city's authorities, who have accused him of endangering national security.

Singapore has an extradition treaty with Hong Kong.

"Mr Law's entry into and presence in the country would not be in Singapore's national interests," said the ministry's spokesman in response to queries from the BBC.

"A visa holder is still subject to further checks at point of entry into the country. That is what happened with Nathan Law," said the spokesman, adding that Mr Law was referred for "questioning, and immigration and security assessment" after he landed.

The government had previously said it "takes a clear and strong stand against the importation of politics of other countries into Singapore".

In a statement, MrLaw said he believesthe denial of entry was for "political" reasons. "I am unsure whether external forces, such as the PRC (People's Republic of China), are involved, directly or indirectly," he said.

Mr Law said he applied for a visa that would have allowed him a "one-time entry for a few days",and that it was approved three weeks before his departure. He said he holds a UK Refugee Travel Document.

On Sunday, the activist was put on the earliest flight back to San Francisco, where he initially departed from.

The organisers of the event he was due to attend have declined to comment to the BBC.

The BBC has also reached out to the Hong Kong and British authorities for comment.

During its routine press briefing on Monday, a Chinese foreign ministry spokesperson said "each country has the right to manage entry and exit on its own."

He also described Mr Law as "an anti-China, anti-Hong Kong troublemaker who has been lawfully placed on the wanted list by the Hong Kong police".

Mr Law, a former Hong Kong legislator whois one of the most prominent figures in the city's pro-democracy movement, fled the territory in 2020 after China imposed a sweeping national security law which targets secession, subversion and terrorism with punishments of up to life in prison.

In 2021, he was granted asylum in the UK.

Hong Kong authorities have offered rewards of HK$1m ($128,000; £95,000) for information that can help them arrest Mr Law and other pro-democracy activists.

This is not the first time Singapore authorities have taken action regarding Hong Kong pro-democracy activists.

In 2019, the city-state fined a Singaporean activist for holding an online forum several years earlier that featured prominent activist Joshua Wong speaking in a teleconference call.