Hong Kong political leaders on Sunday slammed a so-called Legislative Council primary election held by the opposition camp over the weekend, saying the move was unfair to other candidates and may have violated the city's electoral law.

Wong Kwok-hing, a veteran pro-establishment politician who became a district councilor in 1991 and was a LegCo member from 2004 to 2016, described the opposition camp's move is "jumping the gun" and "unfair" to other candidates who will run in the LegCo elections scheduled for Sept 6.

The city's opposition camp held the event a week before the official LegCo candidate nomination period starts. A series of pre-election forums was held before the weekend, allowing opposition candidates of different districts to introduce their election platforms. They also set up street booths and handed out campaign leaflets.

Wong urged the Electoral Affairs Commission and the Registration and Electoral Office to follow the case and investigate whether the acts violated the Elections Ordinance. He said the EAC also needs to require the opposition camp to declare the costs associated with the primary event as a part of its final candidates' LegCo election expense reports.

Lau Siu-kai, vice-president of the Chinese Association of Hong Kong and Macao Studies, said the so-called primary might be considered cheating, because it informed residents about the opposition camp's potential candidates and their election platforms in advance.

The EAC should mull relevant measures to ensure fairness in the elections, including declaring the cost of the so-called primary into the final candidates' election expenses, the sociologist from the nation's leading think tank on Hong Kong affairs said.

During a press briefing, the opposition camp said it would reject the Hong Kong administration's proposed budget if it wins a majority in September.

LegCo House Committee Chairperson Starry Lee Wai-king said such a declaration is "irresponsible," as they are talking about voting down the budget before they know what's in it.

"What they really want to do is to hijack the legislature to achieve their political aims", Lee said during a one-on-one interview with China Daily on Friday.

Lee, who is also the head of the Democratic Alliance for the Betterment and Progress of Hong Kong, said that the move will pose a serious risk to the city if the opposition wins more than half of the seats in LegCo.

She worried that public hospital operations will be disrupted and public services will grind to a halt if the opposition camp votes down all administration-proposed bills, paralyzing the government.