A SMALL group of Hong Kong pro-democracy protesters have broken into the city's legislature as tensions in the Chinese-controlled city escalated following a period of calm.

The flare-up came just hours after court bailiffs managed to clear part of a protest camp in the heart of the city that has been occupied by pro-democracy demonstrators for nearly two months, while leaving most of the main protest site intact.

About 100 riot police with helmets, batons and shields stood guard outside the government building, facing off with protesters demanding free elections for the city's next leader in 2017.

Four people, aged between 18 and 24, were arrested while three officers were treated in hospital for injuries.

It was the first time protesters had broken into a key public building, defying the expectations of many political analysts who had predicted Hong Kong's most tenacious and protracted protest movement would slowly wind down. A small group of protesters charged towards the legislature and used metal barricades and concrete tiles to ram a glass side door. They eventually smashed through, with several managing to get inside.

Scores of riot police, some with shields and helmets, rushed over, using pepper spray and batons to keep other demonstrators from also smashing their way in.

A pro-democracy lawmaker at the scene, Fernando Cheung, said he and other protesters had tried to stop the activists from breaking through.

He added: "This is something we don't want to see happen because the movement so far has been very peaceful."