Malaysian immigration authorities have detained Hong Kong student activist Joshua Wong Chi-fung after he arrived at Penang International Airport on Tuesday morning. He was deported back to Hong Kong soon afterwards.

"This morning we stopped Hong Kong activist Joshua Wong from entering Penang. We subsequently deported him back to Hong Kong on the same Dragonair flight," an immigration official at Penang airport told The Straits Times.

Wong, 18, was invited to Malaysia to take part in academic talks about the city's pro-democracy movement and the anniversary of China's Tiananmen Square massacre.

The immigration department said in a statement that Wong was deported because he has been blacklisted.

"During immigration checks, the officers on duty found that he has been listed among those barred from entering the country," Immigration Department director-general Datuk Mustafa Ibrahim said, according to The Malay Mail Online.

When speaking to media at the arrival hall of the Hong Kong International Airport, Wong said the denied entry was totally unexpected, South China Morning Post reported. Wong further added that he deeply regretted the Malaysian authorities' decision.

Joshua Wong is convenor of pro-democracy student group Scholarism, which has spearheaded anti-government protests in Hong Kong. He was a key student leader at Occupy Central protests in Hong Kong last year.

Scholarism said that Joshua Wong attempted to enter Malaysia and was rejected by the Malaysian immigration earlier this afternoon.

"Wong was invited by social activists in Malaysia for a series of public forum later this week, but as he reached Penang, his passport was taken away by the immigration officer, and was later informed that his entry to Malaysia has been rejected, without explaining the reason behind such rejection. Wong was required to return to Hong Kong by the same flight," Scholarism wrote in a statement posted on its Facebook page.

Reacting to the news of Wong's deportation, Malaysian home minister Ahmad Zahid Hamidi said that visitors who are deported from Malaysia are usually sent back on grounds of national security.

"There are a few SOP (standard operating procedures) we have to look into, mainly national security, of course," Zahid said, according to Malay Mail Online. The home minister also warned that "stern action" would be taken on anyone who brings "the element of negativity."

The socialist party of Malaysia has condemned the move, accusing Malaysian government of kowtowing to Beijing.

"There is no reason for stopping the pro-democracy activist from entering our country except if the government is kowtowing to the pressure of the Beijing Government which has been barring activists of the Umbrella Movement in Hong Kong from entering mainland China," it said in a statement, Free Malaysia Today reported.