Hong Kong pro-democracy protest leader Joshua Wong and two other leaders from the student group Scholarism have gone on hunger strike starting immediately, they announced on Monday night.

"Living in these troubled times, there is a duty. Today we are willing to pay the price," the students wrote in a declaration on Facebook, after announcing the hunger strike on stage to demonstrators at Hong Kong's main Admiralty protest camp.

Wong and female leaders Lo Yin-wai and Wong Tsz-yuet, 17, said they are fasting to attempt to force the Hong Kong government to respond to their demands for free elections in the semi-autonomous Chinese city in 2017.

"Our future, we will take it back," they wrote.

The trio are calling on Hong Kong authorities to reopen stalled talks with students and for the Chinese government to withdraw its decision to vet candidates for leadership of the city.

They will drink only water during the strike, a local television station reported.

It comes as the two-month-old "Umbrella Movement" strives to find new ways to push forward its demands for full democracy, as Hong Kong authorities order the clearance of key protest zones.

The previous 24 hours witnessed some of the worst violence to hit the protests in weeks, with police using pepper spray and batons on students trying to storm government headquarters.