Thousands of pro-government protesters have rallied outside the government headquarters in Hong Kong as tensions rose on the eve of what is expected to be Hong Kong’s fourth mass protest in a month against a controversial extradition law.

While top officials attend a ceremony on 1 July every year to mark the anniversary of Hong Kong’s handover of sovereignty from the UK to China, a large anti-government protest also takes place on the same day.

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A large turnout is expected on Monday, the 22nd anniversary of the handover, as the city is rocked by its biggest political crisis in decades. Millions have taken to the streets to protest against the proposed law allowing for the extradition of individuals for trial in mainland China, where the opaque court system is controlled by the Communist party.

The protests throughout June have been largely peaceful, but on 12 June, the scene turned violent when police used rubber bullets, teargas, pepper spray and batons to disperse crowds and some protesters threw objects back at the police. Protesters have also surrounded the police headquarters twice in past weeks in noisy and emotionally charged protests in which young people spray-painted obscenities on its front wall and barricaded its entrances.

Police set up giant water-filled barricades in anticipation of overnight rallies by anti-extradition protesters to lock down the area where the handover anniversary ceremony is to take place.

Facebook Twitter Pinterest Police erect barriers around the Golden Bauhinia Square. Photograph: Ritchie B Tongo/EPA

The pro-government protesters – mostly middle-aged and older people arriving on coaches – gathered at Tamar Park outside the Legislative Council and government headquarters from Sunday afternoon. Mostly dressed in blue and white, they waved Chinese national flags and placards with messages supporting the police, who have been under fire for their use of teargas and rubber bullets in anti-extradition protests.

Angry exchanges took place on both sides when pro- and anti-government protesters met. “We were completely besieged by thousands of them at one point. They were very emotional – it felt really dangerous,” said an anti-extradition protester who gave his name as Jim.

A number of journalists were spat at, had mud slung at them, and were kicked and insulted by protesters who accused them of being sympathetic to earlier anti-extradition protests.

Facebook Twitter Pinterest Government supporters shout at the pro-democracy activist Joshua Wong, right, at the Legislative Council complex. Photograph: Kin Cheung/AP

A pro-democracy lawmaker, Lam Cheuk-ting, was surrounded by dozens of protesters on a walkway near the legislature, some of whom used umbrellas or placards to assault him and tried to snatch his mobile phone.

On early Sunday evening, groups of young anti-extradition protesters, mostly dressed in black, were stopped by police from going near the legislature. Police held their identity cards while ordering them to line up to wait to have their bags searched. Police officers questioned them loudly while taking videos of them and took their personal details before letting them go.

Much of the artwork posted on walls near the legislature by anti-extradition protesters was torn down by government supporters as they passed.