Independent lawmaker Wong Yuk-man was arrested last night on suspicion of common assault after he hurled a glass in the direction of Chief Executive Leung Chun-ying in a Legislative Council meeting on Thursday.
The arrest came on the same day as five members of the July 1 pro-democracy march were held for leading the rally too slowly.
The arrests were seen as a sign the government may be toughening its stance in the face of mounting pressure for "genuine" universal suffrage.
Arriving at the police station in Central by car, Wong flashed a "victory" sign to a group of supporters and reporters who had been waiting for him. He was accompanied by a lawyer.
"I got a tip-off that the police want to arrest me. Save the trouble - it's just a glass. I am coming here myself. This is how you act like a hero," he said.
"If they really lay a charge against me, I must make 689 appear in court!" he said, in a mocking reference to the number of votes the chief executive received from the 1,193-strong Election Committee to win the election two years ago.
Police said Wong, 62, was arrested for common assault and released on bail pending further investigation. A police source said officers went to Wong's office in the afternoon but could not find him. They waited for Wong outside the building while he was in a Legco finance committee meeting.
The incident took place on Thursday morning when Leung arrived in the Legco chamber for a question-and-answer session.
About 20 pan-democrats marched towards him to urge him to take the huge turnout at the democracy rally seriously.
They shouted: "Genuine universal suffrage without screening ... Listen to the people's voice, 689." Wong then hurled documents and a glass in the direction of the chief executive. Leung calmly picked up pieces of the glass and criticised Wong's behaviour. His office later called the police to investigate the matter.
While Wong remained unapologetic, saying he did "not have to be polite to a dictatorship", other pan-democrats condemned Wong's act, saying it was not part of their plan to protest against the chief executive.
Wong's arrest raises the possibility of him losing his seat in Legco. This would happen only if he is convicted and sentenced to one month in jail or more, and if a motion calling on him to step down is passed by two-thirds of his colleagues in Legco, under article 79 of the Basic Law.
Barrister Albert Luk Wai-hung said whether Wong had actually injured anyone would be an important consideration if he is charged and a court must rule on his guilt or innocence. It would also be a factor in sentencing.
Luk said it was unlikely a first-time offender would receive a prison sentence.
Earlier yesterday the five members of the group organising Tuesday's march, the Civil Human Rights Front, were arrested for "failing to comply with directions given by police officers" and obstructing police.
Officers said the truck leading the procession moved at snail's pace to deliberately slow down the rally. Another 511 people were detained after the march for staging a sit-in in Chater Road.