Hong Kong student leader Joshua Wong, the teenage face of the city's pro-democracy protests, was assaulted in the street with his girlfriend in an attack he said Monday sent a "chill to my heart".

The motivation for the assault is not known but previous attacks against prominent media figures have raised concerns that tensions from Hong Kong's deep political divisions could turn violent.

Wong, 18, was leaving a cinema near Mong Kok -- the scene of some of the most bitter clashes in last year's street rallies -- with his girlfriend late Sunday when the assault occurred.

The male attacker punched Wong in the face and when he and his girlfriend gave chase both were assaulted, he said on his Facebook page.

"Being attacked on the way home after going on a date, and even attacking my girlfriend, it's shameful," he said.

Police said Monday they had yet to make an arrest, confirming that Wong had "suddenly been attacked" by a suspect in his 20s and had sustained injuries to his eyes and nose.

"The suspect tried to flee but the two victims followed, and when the female tried to take pictures of the suspect he then attacked the male and female victims," a police spokeswoman said.

Last year's mass protests were sparked after Beijing insisted that candidates for Hong Kong's next leader must be vetted by a loyalist committee, a decision campaigners including Wong derided as "fake democracy".

The electoral proposals were voted down earlier this month after a protracted debate that divided the city.

"(The assault) implies activists are facing the danger of attacks in their daily lives, not only during protests. This is what sends a chill to my heart," Wong wrote in an emotional Facebook post.

"It's not only a problem with universal suffrage -- it's about the limited freedom and legal system slowly being obliterated by these violent acts.

"The road ahead is long and tough, but we should retain our goal and keep walking on this bumpy road of democracy."

- Polarised city -

Other leading anti-establishment figures targeted in the past include media tycoon Jimmy Lai, whose office and home were firebombed in January.

Kevin Lau, former editor of the liberal Ming Pao newspaper, was attacked by knife-wielding assailants in February last year.

Separately, scuffles broke out between what police described as rival protest groups in Mong Kok Sunday night. Five people were arrested after police used pepper spray to separate the groups, the South China Morning Post reported.

The newspaper said the conflict broke out between a pro-Beijing group and "anti-mainland demonstrators".

Political analyst Sonny Lo said that Hong Kong was becoming increasingly polarised with small groups becoming "highly politicised by an increasingly radicalised environment".

"We have now entered the post-Occupy movement stage in which violent confrontations between the two camps, namely the pro-democracy and the pro-Beijing groups, appear to be inevitable," he said, referring to the mass street protests known as the Occupy or Umbrella Movement.

The incidents come days before a major pro-democracy march on Wednesday, when thousands are expected to take to the street.

But organisers distanced themselves from radical groups.

"For a couple of years there have been opposing groups protesting alongside the main march (but) we have been able to maintain the order," said Johnson Yeung of march organisers Civil Human Rights Front.