Zhang Xiaoming, head of the city's mainland China liaison office, was invited to the lunch by a pro-Beijing lawmaker following the July 1 protest in a bid to improve dialogue between the two sides.
Several members of the feisty pro-democracy camp walked out after voicing their protests to Zhang at the meeting in the city's Legislative Council -- the first time a mainland official has attended a formal function at the assembly.
Beijing has promised to hold democratic elections for the city's chief executive by 2017, but critics have cast doubt on the timetable and say the former British colony's political future is in doubt.
The pro-reform lobby fears that even if elections are held by 2017 as promised, Beijing will try to weaken the influence of the democrats, who have dominated previous elections in the city of seven million.
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Beijing has often refused to meet Hong Kong democrats in the past due to their criticism of the Tiananmen Square crackdown in 1989, with some of them banned from visiting the mainland.
"My demand is the release of Liu Xiaobo and that the Communist Party in China end one-party rule," Leung said before the lunch.
Zhang, however, said he hoped more meetings would follow.
"I think this lunch is only a beginning, I believe more dialogue will come," Zhang told reporters after the lunch.
On July 1, tens of thousands of protesters marched under torrential rain demanding democracy and denouncing Chief Executive Leung Chun-ying as a puppet of the mainland.