A 70-year-old man became the second to die in a week of unrest in Hong Kong yesterday, after he was reportedly hit with a brick during a clash between pro-democracy protesters and government supporters.

The unnamed man was using a lunch break from his cleaning job to clear bricks from the streets that had been thrown in protests when he was caught up in a fresh outbreak of violence.

Protests in Hong Kong, now in their sixth month, have turned more violent turn this week after the death of a student who fell from a car park during a police operation last Friday.

Students at a Hong Kong university last night sealed the exits to search for undercover police as they fortified barricades, setting the stage for another round of violent clashes.

At the Hong Kong Polytechnic University, one of three barricaded by pro-democracy demonstrators, scores of students prepared for confrontation with riot police by stockpiling makeshift weapons including petrol bombs.

Pictures of suspected undercover police were passed around by students and on-the-spot searches carried out amid heightened paranoia that authorities were planning to break the campus sieges that began earlier this week.

The developments came as China's President Xi Jinping warned that protests threaten Hong Kong's "one country, two systems" principle governing the semi-autonomous city.