With work scheduled this year on seven new and existing subway lines, the city's traffic would face 'tough times', the China Daily quoted Mayor Han Zheng as saying on Monday.

'With so much work being carried out at the same time and long lengths of new tracks being put into use, both the construction and operation of Shanghai's subway have reached an extreme level,' Mr Han said. 'I am really worried about its safety.'

Subway work, including the building of 116 stations, would affect more than 1,000 roads in Shanghai this year, the newspaper said.

Mr Han's comments come after a train on a subway line under the Huangpu River broke down between two stations during peak hour on Friday morning, leaving passengers gasping for air.

'It was all dark and there was no ventilation. People were scared and a woman said she was suffocating,' the paper quoted Mr Bu Chunyuan, a journalist on the train, as saying.

'Some broke the door and the ceiling by force to let in some air,' Mr Bo said.

Shanghai plans to extend its subway to cover 510km by 2012, from its current coverage of 234km, to ease passenger loads on the network which has been plagued with delays in recent years.

Three officials were arrested in connection with a collapsed subway tunnel under the Huangpu in 2004, which caused floods and nearby land and buildings to sink, costing the city 850 million yuan (S$168 million). -- REUTERS