DHAKA (AFP) - Almost 140 Bangladeshi garment factories were shut on Monday as thousands of workers protesting a new minimum wage clashed with police outside Dhaka, police and manufacturers said.

Protests at poor wages and working conditions have gained in intensity since the collapse of the Rana Plaza factory complex in April, which killed 1,135 people.

Police said violence erupted at Ashulia, home to Bangladesh's biggest garment plants which make clothing for top Western retailers such as Walmart, at Konabari in the industrial district of Gazipur north of Dhaka and in other parts of Gazipur.

The trigger for the new protests was worker unhappiness at the new minimum wage the government has announced for the country's four million garment workers.

Although the minimum monthly salary for entry-level workers has been raised by 76 per cent to US$68 (S$85) as of December, unions have complained that skilled employees have been deprived, while some bosses cut food and transport allowances.

The new wages still leave Bangladeshi garment workers as some of the lowest paid in the sector worldwide.

"At least 10,000 workers demonstrated at Konabari. They threw rocks at the officers. We fired rubber bullets and tear gas," Mr Shamsur Rahman, a spokesman for the industrial police, told AFP.

Mr Rahman said the new wage scale has angered senior sewing operators who said their pay differentials have been downgraded.

Mr Shahidullah Azim, vice-president of the Bangladesh Garment Manufacturers and Exporters Association, said Monday's protests forced the closure of 87 factories at Ashulia, 37 at Konabari and around 15 more elsewhere in Gazipur.

He rejected the skilled workers' complaints.

But union leader Babul Akter said the new pay scales treated many workers unfairly.

"The workers are angry the government has not kept its word to make the new wage effective from Sunday. Skilled workers are not happy because they did not get the same hike as entry-level workers," he said.

"Besides, in some factories, the new pay scale will lead to a cut in food and transport allowances. In the new scale, the annual wage increment has been fixed at five percent while workers in many factories already get annual increments of eight to 15 per cent," he added.

The manufacturers' association agreed last week to the new minimum monthly wage following days of unrest in Ashulia and elsewhere which closed hundreds of factories.

While pro-government labour unions have accepted the 76 per cent rise in the minimum wage, left-leaning labour groups have rejected the deal and demanded a US$100 minimum.