Shenzhen Mayor Xu Zongheng has blamed the city's notorious public security problems on the huge migrant population and cross-border traffic. He levelled the accusations at a National People's Congress press conference in Beijing yesterday. Mr Xu said the large floating population and a lack of experience in social administration made Shenzhen one of the most difficult cities to manage. He did not directly answer questions on how the municipal government would protect Hongkongers' safety. But he promised that 'necessary measures will be launched' to improve citizens' and travellers' sense of security. 'To improve Shenzhen's public security will be a long-lasting and arduous task,' he said, referring to a situation where migrants far outnumber permanent residents. 'Only 2 million of the city's population of 14 million have permanent resident permits.' He said the floating population had fewer social responsibilities than permanent residents. Shenzhen is home to 7 per cent of the nation's migrant workers, and mainland media have reported that nearly 80 per cent of the city's crimes are committed by migrants. Last year the city launched a controversial hi-tech residency card to track floating population movements, which activists argued would erode migrants' civil rights. Mr Xu also attributed the public security situation to busy cross-border traffic between Shenzhen and Hong Kong. 'More than 170 million people and 16 million vehicles departed from China last year using Shenzhen's checkpoints, accounting for 51 and 77 per cent respectively of the country's total flow rate. 'Shenzhen has the busiest checkpoints on the mainland and even the world. It's likely that international or cross-border offenders can exploit [any loopholes] to commit crimes, or even make Shenzhen their criminal base.' Mr Xu said Shenzhen was an 'overnight city' that lagged behind in administrating a huge floating population, and required experience from international cities such as Hong Kong and Singapore.