Two police officers were beaten bloody by pro-democracy protesters in Hong Kong Sunday as violence erupted at a rally calling for greater democratic freedoms in the heart of the city.

Trouble flared when police ordered the authorised gathering to disperse after officers conducting stop and searches on nearby streets had water bottles and paint thrown at them by angry crowds.

A group of plainclothes officers who were speaking with organisers were then set upon by masked protesters, who beat them with umbrellas and sticks, an AFP reporter on the scene said.

Two officers were seen with bloody head wounds as colleagues shielded them from further attacks.

"We strongly condemn all the rioters and violent acts," police spokesman Ng Lok-chun told reporters.

Video posted online showed an organiser with a microphone asking the officers to show their warrant cards which they did not do, a frequent gripe among protesters.

Rally organiser Ventus Lau said he believed police should "shoulder the greatest responsibility for the clashes" because they took too long to show their warrant cards.

Lau was later arrested for obstructing officers, police and rally organisers confirmed.

Soon after the officers were attacked, riot police swept into the area and fired tear gas to disperse the crowds.

Brief cat and mouse clashes ensued with police making multiple arrests, including one protester who had blood streaming from the back of his head.

- 'Stand with Hong Kong' -

Hong Kong's protests have raged for seven months after being sparked by a now-abandoned proposal to allow extraditions to the authoritarian mainland, where the opaque legal system answers to the Communist Party.

Riot police swept into the area and fired tear gas to disperse the crowds / © AFP

They soon morphed into a wider movement calling for greater freedoms in what is the most concerted challenge to Beijing's rule since the former British colony's 1997 handover.

At Sunday's rally, thousands gathered in the heart of the Central commercial district, chanting slogans such as "Stand with Hong Kong, fight for freedom".

Some waved American, British and Hong Kong independence flags. There were many families and children present with a peaceful atmosphere until police ordered the crowds to leave.

The frequency and ferocity of Hong Kong's protests have died down over the last month, but signs of the political unrest are everywhere, from graffiti daubed on walls to huge fences surrounding government buildings.

The city's police force is now loathed by large swathes of the city, heckled by crowds both at protest sites and in their local neighbourhoods.

Thousands gathered in the heart of the Central commercial district, chanting slogans such as 'Stand with Hong Kong, fight for freedom' / © AFP

Critics accuse police of using excessive force, with no police officer disciplined or punished in the last seven months of protests.

Police say they have used force commensurate with the levels of violence they face from hardcore protesters who routinely throw bricks and petrol bombs.

The force has blamed viral social media videos of officers making hard arrests and media coverage for their plummeting reputation among the city's inhabitants.

Among key demands of the protest movement are an independent inquiry into the police, an amnesty for 7,000 people arrested and fully free elections.

Beijing and local leader Carrie Lam have refused further concessions and defended police tactics.