Imprisoned Hong Kong dissident Joshua Wong has been handed an additional 10-month sentence on after he pleaded guilty to taking part in an "unlawful" protest last year over the Tiananmen Square crackdown.

Hong Kong has regularly marked the anniversary of Beijing's deadly 1989 repression of protests in Tiananmen Square with huge candlelight vigils.

But last year's event was banned for the first time, with police citing the coronavirus pandemic and security fears following huge democracy protests that roiled Hong Kong the year before.

Tens of thousands defied the ban and massed peacefully at the vigil's traditional site in Victoria Park.

Since then prosecutors have brought charges against more than two dozen prominent democracy activists who showed up at the vigil, the latest in a string of criminal cases that have ensnared the city's beleaguered democracy movement.

Today, four of those activists - Joshua Wong, Lester Shum, Tiffany Yuen and Janelle Leung - were handed jail terms after pleading guilty to unlawful assembly charges last month.

Wong, one of the most recognisable faces of Hong Kong's democracy movement, is currently serving a total of 17-and-a-half months in jail for two convictions linked to the 2019 protests.

A judge handed the 24-year-old a consecutive ten months of jail for the new conviction which will start once current sentences are finished.

"The sentence should deter people from offending and reoffending in the future," the judge said.

Shum, 27, was given six months while Yuen, 27, and Leung, 26, were both handed four months.

Wong, Shum and Yuen have also been charged under a new national security law Beijing imposed on the city last year.

Ahead of today's sentencing they were being held in pre-trial detention and face up to life in prison if convicted under the new security law.

The other defendants, who include some of the city's most prominent activists, many of them also jailed or in detention, will be tried later this summer.