And despite a huge increase in seizures of pull-up posters and other roadside promotional items - almost 16,000 last year compared to a few more than 8,000 the previous year - there have been few convictions.

Two laws, one against obstructing streets, the other prohibiting the display of posters without permission, are used to control the salespeople's behaviour.

But frontline officers who must enforce the law say legal loopholes, poor allocation of staff and failure to prosecute the mobile phone and pay-TV companies behind the advertising make it almost impossible to get a conviction.

'Currently the government mostly uses the legislation against the display of posters to go after the salesmen,' the chairwoman of the Food and Environmental Hygiene Department Staff Rights Union, Li Mei-siu, said.

'To get a conviction in court with this legislation, you have to prove that a defendant has engaged in 'displaying' the materials. When you confront the salesmen, they always say they just happen to be standing next to the pull-up poster.

'To get a conviction, you need to lie in wait early in the morning to catch them unfurling the posters. This is, of course, impossible and ridiculous to do.'

A total of 15,664 seizures of roadside advertising materials were reported last year, an increase of 87 per cent on 8,377 in 2008.

Individual districts showed even greater rises, with Yuen Long reporting a 2,727 per cent increase in such seizures and Central and Western District 843 per cent. The government attributed these increases to a crackdown launched in Wan Chai and Mong Kok in October 2008 and later extended to eight other districts.

Tsuen Wan and Mong Kok are the most cluttered districts, recording 6,155 and 3,583 confiscations respectively last year. With an average of seven pull-up posters seized every day over the past year, the Sai Yeung Choi Street South pedestrian precinct is the most clogged street in Mong Kok.

Despite these numbers, convictions are scarce.

In the past two years, only 23 convictions using the display legislation have been recorded, even though 24,041 seizures have been made and 17,038 verbal warnings issued.

'In most cases, salesmen will disperse and abandon the frames at the sight of our staff,' Li said. 'That's why the conviction number using the display legislation is minuscule compared with the seizures.'

With 1,699 convictions in the past two years, the legislation against street obstruction seems to be more effective.

But Li said a recent manpower reshuffle by the government has hamstrung their crackdown.

About 3,000 staff of the department's hawker control team were previously used to pursue the salespeople under the obstruction legislation.

But in 2009 its cleaning team, comprising just 360 people, took up the bulk of the work.

'The cleaning team can only use the display legislation to go after the salesmen, but under the display legislation it is next-to-impossible to secure a conviction in court,' Li said.

Lack of communication between cleaning and hawker control teams in joint operations further hobbled the clampdown, Li said.

'The hawker team always steps aside and leaves the cleaning team to confront the salesmen on their own,' she said.

'Frequent arguments break out between the cleaning team and salesmen, who believe the cleaning staff overstep the mark in doing the job supposed to be done by the hawker team.'

Li said that when a conviction was achieved, poorly paid sales staff were often left carrying the can for their wealthy employers, paying fines ranging from HK$80 to HK$1,500.

'They are very poor. They just earn meagre commissions,' she said. 'Some companies will help pay the fine once or twice. Afterwards, they are left to their own devices.'

For their part, the streetwise salespeople have devised slick tactics to fend off prosecution. One working for PCCW said he quickly pulled down his poster when he saw an officer.

'Those frames are erected with a pole. You take the pole out and the poster will automatically roll into a small scroll in seconds. [Then] the frame and the pole are just our personal belongings,' he said.

Tsuen Wan district councillor Richard Chan Kam-lam said the problem could only be solved by prosecuting the service providers.

'I have received many complaints,' he said. 'Shop owners say the frames block their view. Pedestrians find them a nuisance. Others say their spillover onto roads blocks traffic.'

Democratic Party legislator Fred Li Wah-ming said there was no specific legislation in place aimed at cracking down on the practice.

'The government should step up their patrols to keep the practice in check,' he said.

The Food and Environmental Hygiene Department declined to explain the reason for the manpower reshuffle. It said it would seek to convict service providers if there was sufficient evidence.