They should consider installing surveillance cameras on all streets in the pedestrians-only zone around the lake and electronic displays at the entrances to show images of everyone caught littering, it said.

"We are considering if this is necessary," said Pham Tuan Long, vice chairman of Hoan Kiem District.

But he added the district has some fixed cameras and would install more to record litterbugs.

Hanoi Urban Environment Company (URENCO) said after it installed two cameras and sent 30 staff to record litterers a month ago the number of litterbugs around the lake decreased dramatically.

"However, using staff to record violators is not a long-term solution because it is a waste of human resources," a company spokesperson said.

"They may also face many risks and have to deal with overreaction by violators."

During the three-week trial it recorded clips showing six litterers who paid a total fine of VND13 million ($558) and submitted them to local authorities.

The laws provide for fines of VND5-7 million ($215-300) for littering sidewalks, streets or the drainage system, but they are rarely evoked.

The streets around the lake are for walking only from 7 p.m. to midnight on Saturdays and Sundays.

District authorities say they receive 20,000-25,000 visitors each day and the number goes up to around 200,000 during holidays and festivals.

The streets are badly trashed, especially when they are used as a venue for outdoor events.

URENCO said it has been collecting 200 tons of garbage every day from the pedestrians-only area.