Thousands of people have taken to the streets of central Hong Kong holding candles as they marched in memory of pro-democracy Chinese dissident Liu Xiaobo.

Liu's ashes were buried at sea on Saturday, depriving supporters of a place to pay tribute following his death on Thursday from cancer while in China's custody.

Veteran activists led the solemn gathering later on Saturday which wove its way from the commercial heart of Hong Kong island to China's liaison office, with some marchers in tears.

People in Hong Kong held memorial events but Saturday's was by far the largest.

Marchers carried floral wreaths and white chrysanthemums, bowing three times in front of a makeshift memorial to Liu outside the liaison office, a traditional sign of respect at funerals.

Among the crowd was Lam Wing-kee, one of the five Hong Kong booksellers known for publishing salacious titles about Beijing politicians who vanished and resurfaced in the mainland in 2015.

Lam was allowed back to Hong Kong last year on condition that he pick up a hard disk listing bookstore customers and return.

Lam said he had been saddened by the news of Liu's death but would not lose hope for the future of Hong Kong.

"If the timeframe you look at is too short, it will be pessimistic," he told AFP news agency. "It's not enough just to see what's in front of you."

'Great scholar'

Steven Wong, 45, had travelled from Singapore to attend the march, saying he had respected Liu for many years.

Wong was born and grew up in Beijing and was a high school student there in 1989, the year of the crackdown on pro-democracy protesters in the city's Tiananmen Square.

He said he remembered burned-out tanks and blood smeared on lamp posts the day after. Shortly afterwards his family moved to Singapore.

"He was a great scholar who woke up young people, especially of my generation," Wong said of Liu.

"He made me think deeply about what we can do as a Chinese [person] and what we can teach our students," said Wong.