Leung Kwok-hung -- known as "Long Hair" -- of the League of Social Democrats was turned away from Kuala Lumpur airport and sent back to Hong Kong.
Leung, 59, had been due to speak about Tiananmen Square and Hong Kong's electoral reforms to local activists today night with an open discussion on human rights scheduled for Saturday.
"We do not welcome troublemakers into Malaysia and that is why we deported him (Leung) back to Hong Kong. We only welcome peace-loving people," Inspector-General of Police Khalid Abu Bakar told AFP.
"The fiasco is unfounded and unnecessary," said Avery Ng, the vice chairman of the League of Social Democrats, who works closely with Leung.
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"This act shows that the Malaysian government does not respect freedom of speech."
Hong Kong's security minister Lai Tung-kwok said the city's authorities would not intervene.
"We respect and we do not intervene in decisions made by immigration authorities in accordance with their law," he told reporters.
The Malaysia-based group that had arranged Leung's visit said he had told them by phone that his passport had been seized by immigration officials at Kuala Lumpur airport and he had been detained for 90 minutes, before being sent back to Hong Kong on a Cathay Pacific flight.
"I suspect... Similar to the recent deportation of Joshua Wong, China could have put pressure on Malaysia to ban Leung from entering.
"We demand Beijing and Kuala Lumpur explain why the pair are not allowed into Malaysia," he said.