"Long Hair" Leung Kwok-hung's Legco seat is at risk again, less than three weeks after he was re-elected.
The lawmaker and five other activists admitted charges related to blocking roads during last year's July 1 pro-democracy march.
A prison sentence of more than one month would open up the possibility of a new attempt to impeach Leung.
Legco can expel any lawmaker who is jailed for more than one month, subject to a two-thirds majority in favour.
Leung and Cheung Kam-hung, Chow Nok-hang, Leung Wing-lai, Hung Hiu-han and Shum Tse-kit pleaded guilty to holding a public meeting without notifying police, in breach of the Public Order Ordinance, and unlawful assembly.
Sentencing at Kwun Tong Court was adjourned until today.
In April, Leung survived an impeachment attempt after he was given a two-month jail sentence for being part of a crowd that disrupted a public forum on plans to scrap by-elections.
After a seven-hour debate, only 19 out of the 46 lawmakers present supported the motion.
Leung, who topped the poll in New Territories East in the September 9 election, is appealing against that conviction, with a hearing due in November.
In court yesterday, senior public prosecutor Jonathan Man Tak-ho said the police had given the Confederation of Trade Unions permission to hold a demonstration between 2pm and 8pm. But Man said the defendants caused heavy congestion after the authorised time when they occupied the junction of Queen's Road Central and Garden Road, and Connaught Road Central.
"At 11.32pm, marchers at Connaught Road Central eastbound suddenly climbed over the barriers and occupied Connaught Road Central westbound near City Hall," he said.
"The police found that 14 drivers, mainly taxi drivers, were delayed by marchers for an average of 20 to 30 minutes."
Leung told the court he acted "under the pressure of circumstances" but needed more time to prepare his mitigation. Magistrate Don So Man-lung adjourned sentencing until today.
Two other defendants, Chan Sin-yuk and Or Pui-yan pleaded not guilty to the same charges, and the prosecution offered no evidence against them. So ordered that they be bound over in the sum of HK$1,000 for one year.
Outside the court, Leung said the charges he faced - unlawful assembly and violating the Public Order Ordinance - were unconstitutional because Hongkongers have a right to protest. He demanded that the administration scraps the ordinance.
Fellow defendant Hung said: "We pleaded guilty because it's impossible for us to defend against the unconstitutional ordinance."
Former lawmaker Chim Pui-chung lost his seat in 1998 after being sentenced to three years, reduced to 12 months on appeal, for plotting to forge documents.