Bangladesh police have arrested three union leaders over deadly protests against a new minimum wage for garment workers that have forced hundreds of factories to shut, an officer said Wednesday.

Police arrested the trio over their roles in the protests which have left two people dead in key industrial districts outside the capital, where workers stitch clothes for top Western retailers such as Walmart and H&M.

The protests over the new $68 minimum monthly wage erupted last week and forced the closure of some of the country's biggest garment plants, crippling the sector, a mainstay of the nation's economy.

"They were arrested on Tuesday for inciting violence," M. Kamruzzaman, police inspector of Gazipur district told AFP.

One of the leaders arrested, Mohammad Kafiluddin, heads a union based in Gazipur, while another, Ziaul Kabir Khokan, is the Gazipur head of the Trade Union Kendra, a union affiliated with the Communist Party, union officials said. Details of the third leader are unknown.

Tens of thousands of workers have taken to the streets in Gazipur and neighbouring industrial hubs of Ashulia and Savar, clashing with police to protest the new wage fixed by the government.

Two workers were shot dead on Monday and hundreds have been injured in the violence that began after the government announced the 76 percent hike earlier this month.

While pro-government unions and the manufacturers have accepted the new figure, left-leaning groups are demanding a $100 wage, saying that amount is the minimum needed to maintain a decent life.

Protests against poor pay and conditions have gathered momentum since the collapse of the Rana Plaza factory complex in April on the outskirts of Dhaka that killed 1,135 people.

Bangladesh's $22 billion garment industry is the world's second largest after China's and employs four million workers, most of them women.

The new wage is still below that given to workers in other major exporting nations, including Cambodia, Sri Lanka and Vietnam, according to the International Labour Organisation.

Left union leader Moshrefa Mishu, who heads the Garment Workers Unity Forum, said she has been threatened by unknown callers since rejecting the rise.

"They threatened to kill me if I don't stop the protests," she said, adding police have raided the homes of at least 10 Gazipur-based garment union activists belonging to her union.

Mishu was detained for five months in 2010 during one of the country's deadliest unrests against low wages. She still faces 11 charges stemming from those protests.

Unions have been targeted by the country's intelligence agencies in the past. Key union leader Aminul Islam was picked up in 2012 allegedly by plain-clothed policemen and was later found murdered.