A frontline policemen's group 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 ease the alienation between the two sides, and could further escalate the tension.

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 littered bike 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 has 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 actual investigation began 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.'

The association has published a strongly worded statement saying society should reprimand 'the protesters who broke the law during the protest'. It expressed regret that some protesters 'were not respecting the rule of law, and were damaging the city's order by waving their flags of freedom'.

Lawyer Albert Luk Wai-hung said the bike case could be classified as vandalism if the handbills had scratched the bike or if corrosive glue was used. But if banners and flags were simply placed on the vehicle, it would be difficult to prove the crime. As for tampering, the prosecution has to prove a criminal motive, he said.

Disorderly conduct could be involved in the whistle case and the officer, a sergeant, could pursue damages in a civil lawsuit if his hearing was damaged by the protester, Luk said.

Professor Ma Ngok, a political scientist at Chinese University, said public anger against 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.

'That has affected the image of the whole institution. 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.

63,000

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