Security minister Chris Tang said on Wednesday that protests in Hong Kong commemorating the victims of the Urumqi fire last week are signs of what he described as "a colour revolution" that threatens national security.

Sporadic protests have taken place on several university campuses as well as in Central and Yau Ma Tei in recent days.

After attending a Legco meeting, Tang said these events are highly organised, using what he called "anti-China" social media.

He said some slogans chanted by protesters may violate the national security law because they involved the words "revolution, autocracy" and "urging the leader to step down".

"Inside the universities and on the street corners, we observed that there are people using this opportunity to incite against the central government," Tang said.

He also said some people who were involved in the 2019 unrest took part in the recent protests, and that he had to speak out and take measures to prevent a repeat of what happened three years ago.

Tang added that university management are responsible to ensure campuses won't become the base of rioters again.