There were reports of rocks being thrown at trucks attempting to break the picket line, with some windscreens damaged.

The striking drivers are mostly independent hauliers from outside Shanghai, who earn a flat rate on deliveries and have to pay all their own expenses. 'Our haulage fees haven't changed in over a decade but the costs have gone up dramatically,' said one striker, from Henan province . 'The price of fuel has gone up so much that we can hardly afford to feed our families.'

The drivers said they felt frustrated at the lack of response from the Shanghai municipal government.

'We have been striking since Wednesday but the government refuses to speak to us,' said another driver, who like other protesters, refused to give his name. 'Overseas media have been reporting about our situation, but the national newspapers are ignoring us.'

The main strike action was at the entrance to the Shanghai Zhongji Vehicle Logistics Centre in Baoshan, but there were reports of similar blockades at three quays in the Waigaoqiao docks district in the east of the city.

Container trucks were able to enter docks unhindered in Waigaoqiao yesterday afternoon, although traffic appeared relatively light for the usually bustling port. Several police vehicles were parked at each of the entrances to the docks, apparently in anticipation of disturbances.

The drivers of container trucks parked in nearby streets denied they were picketing. 'I know about the strike and I agree with it, but I don't dare get involved,' said one.

Some shipping industry staff said the industrial action had begun to affect their trade. A manager with Shanghai Winspeed Freight, who asked not to be named, said the walk-out had definitely affected business.

'Ships aren't able to leave and freight can't be hauled,' he said. 'Truck drivers can't enter the port or the depot since the entrance is blocked. There are few people working at the port now. Most of our overseas clients know of this incident.'

However, other industry figures played down the impact of the strike.

'There hasn't been any significant effect on our business so far,' said Zhang Yuanfeng, liner branch deputy manager at the China Ocean Shipping Agency. 'If the strike continues in the longer term then it will certainly start to take its toll but for the time being it's too early to say whether that will happen.'

By yesterday afternoon, the Baoshan demonstration had quietened down, with just a handful of strikers maintaining a silent vigil across the street from the logistics centre.

Police had used a freight container as a makeshift gate at the entrance to the road leading to the centre, and a team of officers sat watching from inside a cramped police van.

'There were a lot of people here this morning, but not that many came back after lunch,' said one driver. But he predicted another protest would break out this morning and for days to come.

'We are not going to give in,' he said. 'We're going to continue striking until they listen to us.'