Police are investigating whether protesters are criminally liable for covering a police motorbike with handbills and blowing a whistle close to an officer's ear, during the July 1 march.
Photos of the two incidents, widely circulated on the internet, have stirred heated debate about the behaviour of protesters and police during the march.
One picture shows the motorcycle covered with protest handbills, while another shows an officer leaning backwards as a man blows a whistle close to his ear. The police said yesterday that the cases were being investigated by the Hong Kong Island regional crime unit, but no one had been arrested so far.
A frontline policemen's group, which had earlier voiced regret at the incidents, said many officers were unhappy about the pictures, and felt insulted when protesters swore at them. It said the investigation could help boost morale in the force.
But one political pundit said such a probe would not reduce the alienation between the two sides, and could stoke tensions.
Joe Chan Cho-kwong, chairman of the Junior Police Officers' Association, said frontline staff felt there was a lack of respect from protesters at this year's march, and the officers' reaction when they saw the motorbike was particularly strong, he said.
'A police bike is a symbol of the rule of law, and represents the image of law enforcement units,' he said. 'In 2003, there was much more respect from the protesters ... but this year, they swore at us and spat on us.'
The association had asked the force's management to follow up on the protesters' behaviour, Chan said, and to review the relationship between protesters and the force. But he believed the investigation had begun even before they made their request. The case could involve littering, vandalism and tampering with a vehicle, he said.
'It's basic manners to respect each other. We are used to all kinds of shouting and offensive words when it comes to triad crimes, but when they come out from an average person we can hardly accept that.'
Professor Ma Ngok, a political scientist at Chinese University, said public anger at the police stemmed from the increasingly heavy-handed tactics they have used in rallies.
The situation deteriorated after Andy Tsang Wai-hung became police chief, he said.
'It can become a spiral: when the force deploys more policemen in an operation, the protesters become more agitated, and then the police become even more heavy-handed.'
If the police pressed charges, it would create the impression that policemen can be violent without any consequences, while members of the public can be penalised, Ma said.
Police estimate of the number of marchers at the peak of the protest on July 1, although activist groups put the figure at 400,000