Trump Praises 'brave Man' Jimmy Lai

US President Donald Trump on Wednesday called arrested Hong Kong media mogul Jimmy Lai a "brave man" and expressed his support.

"I sent him best wishes," Trump told reporters in the White House, calling Lai "certainly a brave man."

The Apple Daily founder was arrested last week and briefly detained under the SAR's new National Security Law. He was released on police bail.

Trump added that his administration wanted to win back business from Hong Kong, and claimed the SAR's economic success was down to US support.

Trump also fired back angrily at his predecessor Barack Obama after being branded an unserious president who had put US democracy in danger.

Trump told reporters Obama was an "ineffective" and "terrible" leader.

"I see the horror that he's left us, the stupidity of the transactions that he made," Trump said.

"President Obama did not do a good job. The reason I'm here is because of President Obama and Joe Biden," he said.

Obama is due to speak to the Democratic convention later, accusing Trump of never showing "interest in taking the job seriously," according to excerpts of his speech. (AFP)