HONG KONG (AFP) - Unpopular Hong Kong leader Leung Chun Ying gave evidence on Tuesday (April 19) in an assault case against a legislator who threw a glass at him in parliament, the first time a sitting chief executive has testified in court.

A calm Leung occasionally broke into smiles as he was cross-examined by pro-democracy and anti-China lawmaker Raymond Wong, who is accused of common assault and was representing himself in a courtroom packed with supporters from both sides.

Wong has pleaded not guilty to the charge and questioned the validity of allowing Mr Leung's testimony, calling him "a liar and untrustworthy".

The court ruled however that Mr Leung could be admitted as a witness.

It comes as he battles widening political divisions in the semi-autonomous city, with some young campaigners pushing for self-determination or outright independence from China as fears grow that Beijing is tightening its grip.

Rival groups clashed inside Eastern Magistrates' Court before the hearing, with anti-Leung groups shouting: "Down with the Chinese Communist Party," eliciting the response: "May the CCP live for 10,000 years!"

A scuffle broke out when one pro-Leung demonstrator shoved a supporter from the Wong camp, with the Leung group calling their rivals "yellow dogs".

In court, Leung described the incident in July 2014, saying paper had been thrown at him while he was at the podium in the legislature.

"Then I heard glass breaking behind me," he said. "It sounded close and forceful," added Mr Leung, who said he was "startled".

Wong is accused of throwing a glass of water at Mr Leung during the question-and-answer session. It did not hit him, smashing on the floor nearby.

The incident took place as tensions were rising in the wake of a white (policy) paper from Beijing in which it asserted its authority over Hong Kong.

Frustrations later boiled over into massive pro-democracy rallies in September 2014 after Beijing insisted on vetting candidates for the city's future leader ahead of a public vote.

In an hour of detailed cross-examination, Wong questioned Mr Leung over his security detail and the instructions they had been given on the day, as well as whether he knew the identities of those protesting.

Mr Leung said he was unsure which lawmakers were protesting during the session.

The cross-examination will resume on Tuesday afternoon after lunch.

It was the latest public drama for Mr Leung, who is also embroiled in a row over an alleged breach of aviation rules involving the baggage of his daughter.

That led to more than a thousand people protesting at Hong Kong airport on Sunday.

They were angered by an apparent special arrangement that helped 23-year-old Leung Chung Yan after she accidentally left her hand baggage at the check-in counter last month.

Airport staff brought it to her in the restricted zone.

Local media reported the chief executive made a phone call to relevant officers before the bag was delivered to his daughter.

Mr Leung has admitted making a call but denied pressuring staff.