“Jimmy Lai is being arrested for collusion with foreign powers at this time,” Mark Simon wrote on Twitter.
Mr Lai, masked and wearing a blue shirt and a light gray blazer, was led out of his mansion in Kowloon by police officers also wearing surgical masks and was taken away.
Hong Kong police said seven people had been arrested on suspicion of violating the national security law, but the statement did not reveal the names of those arrested.
The police did not rule out further arrests to be made.
Mr Lai, who owns popular tabloid Apple Daily, is an outspoken pro-democracy figure in Hong Kong and regularly criticises China’s authoritarian rule.
The national security law came into effect on June 30 and is widely seen as a means to curb dissent after anti-government protests rocked Hong Kong last year.
Mr Simon said that police searched both Mr Lai and his son’s home, as well as other members of media group Next Digital, which Mr Lai founded.
Over a hundred police also raided Next Digital’s headquarters in Hong Kong, entering the newsroom and searching the desks.
It was not immediately clear how Mr Lai or others at the newspaper may have colluded with foreign forces since the law took effect.
Mr Simon said in a tweet that the police were executing a search warrant.
At times, officers appeared to get into heated exchanges with Next Digital staff present at the scene. Police also cordoned off the headquarters while the raid was being conducted.
Next Digital operates the Apple Daily tabloid, which Mr Lai founded in 1995, ahead of Britain’s handover of Hong Kong to China.
Like Mr Lai, Apple Daily has a strong pro-democracy stance and often urged its readers to take part in pro-democracy protests.
The security law outlaws secessionist, subversive and terrorist acts, as well as collusion with foreign forces in the city’s internal affairs.
The maximum punishment for serious offenders is life imprisonment.
Last month, Chinese broadcaster CCTV said pro-democracy activist Nathan Law and five others were wanted under the law, although all six had fled overseas.
Mr Law had relocated to Britain in July to continue international advocacy work for Hong Kong.
