The months of demonstrations calling for reforms in semi-autonomous Hong Kong crippled its economy and put its leaders and police force under unprecedented pressure.

Lai was picked up from his home by police officers early on Friday, while former pro-democracy legislator Yeung Sum and former legislator and vice chairman of the Labour Party, Lee Cheuk-yan, were also arrested.

The three left the police station after being charged and are to appear in court on May 5. They could face up to five years in prison along with fines.

All three were charged with illegal assembly in connection with the August 31 march, which was timed to mark the fifth anniversary of a decision by China against fully democratic elections in Hong Kong.

Organisers called off the march after police banned it, but hundreds of thousands of people defied the order and filled the streets in several areas of the Asian financial hub. Protesters threw petrol bombs at government headquarters and set fires in the streets, while police stormed a subway car and hit passengers with batons and pepper spray in some of the most violent scenes up to that point in the protest movement.

The US expressed concern over the arrests, with State Department spokeswoman Morgan Ortagus saying Washington expected local authorities "not use law enforcement selectively for political purposes."

Hong Kong's last British governor, Chris Patten, described the three arrested as "brave and respected advocates of free speech, accountable government, responsible social policy and political liberty."

"This decision will send yet another signal to the world that the Chinese Communist Party is intent on throttling decency and freedom in Hong Kong.," Patten said.

China's official Xinhua News Agency, published a scathing criticism of Lai and the others, accusing them of colluding with foreigners.

Australian Associated Press