Thousands of garment workers in Bangladesh protested and vandalised factories on Monday, demanding nearly three-fold minimum wage hikes, as security forces fired tear gas on them, police and union officials said.

Bangladesh is one of the world's largest garment exporters, with the industry accounting for 85 percent of the country's $55 billion annual exports, but conditions are dire for many workers.

Police said at least 10,000 workers left their shifts and staged protests in Gazipur, the country's largest industrial city, and another 7,000 protested in the central towns of Ashulia and Hemayetpur.

Garment union leader of Ashulia region, Mohammad Ibrahim, disputed police figures, saying protesters totaled at least 100,000.

"Protesters vandalised factories and tried to force workers to join", said Sarwar Alam, chief of the industrial police unit in Gazipur, saying at least 40 factories were damaged after protesters smashed windows and damaged furniture.

"We fired tear gas and sound grenades to disperse the workers," Alam told AFP.

Bangladesh is home to around 3,500 garment factories, where clothing for some of the world's largest retailers and brands is made, but the basic monthly wage for workers is just 8,300 taka ($75).

Protests erupted over the weekend after the powerful manufacturers' association offered a 25 percent raise, ignoring union demands for a basic new monthly minimum wage of 23,000 taka ($208).

'Basic needs'

The South Asian country of nearly 170 million has overtaken its neighbor India in per capita income, with the garment industry at the center of its impressive growth over the past two decades.

But wage protests pose a major challenge to Prime Minister Sheikh Hasina, who has ruled the country with an iron fist since 2009.

A resurgent opposition has challenged her rule as she readies for elections due before January.

Her government set up a panel this year to fix a new minimum wage.

But unions say that the manufacturers — who include ministers and influential lawmakers — have played a role in fixing the minimum wage during past negotiations.

"The workers have been badly impacted by a cost of living crisis, and the cost of food has skyrocketed," said Taslima Akter, the head of Garment Sramik Samhati union.

"If you take into account inflation and the depreciation of the taka against the dollar… the manufacturers were offering less than what a worker got in 2017 when the basic minimum wage was fixed."

Police fired water canons, rubber bullets and tear gas at protesters in Ashulia after workers vandalised factories and blocked roads, officers told AFP.

Mahbubur Rahman, police chief of Bangladesh's industrial regions, told AFP they were talking to union leaders to peacefully resolve the protests.

Major brands including Gap, Levi Strauss, Lululemon and Patagonia wrote a letter to Hasina this month calling for a "successful conclusion" to wage negotiations.

"The consultations should seek to raise the minimum wage to a level that corresponds with a wage level and benefits that are sufficient to cover workers' basic needs and some discretionary income," they wrote on October 13.