Thousands of Bangladeshi opposition supporters staged rallies across the country on Monday demanding the release of their jailed leader Khaleda Zia and free and fair elections.

Zia, twice prime minister in the Muslim-majority nation of 160 million, was imprisoned for five years last week for embezzlement, sparking riots from supporters who rejected the verdict as politically motivated.

The leader of the Bangladesh Nationalist Party (BNP) is expected to appeal but the conviction may affect her ability to stand in a general election slated for December.

Thousands of BNP activists in cities across Bangladesh joined hands in a march calling for Zia's immediate release from a prison in Dhaka.

“We want her immediate release and a free and fair election,” BNP secretary general Fakhrul Islam Alamgir told AFP outside the national press club in Dhaka where more than 5,000 gathered.

Zia, 72, is expected to be freed on bail once the appeal has been lodged but her supporters fear Bangladesh's ruling party will try and keep her detained on other charges.

She is accused of dozens of other criminal violations including corruption and violence.

One of her lawyers, Zainul Abedin, said authorities were attempting on Monday to hold Zia in connection with an arson attack in 2015.

Alamgir said at least 4,300 opposition officials had been arrested in the past fortnight as police sought to offset resistance ahead of Zia's sentencing.

A party vice chairman and two former ministers were among those detained, he added. Police have not commented on the arrests.

Zia is a former ally-turned-archrival of Prime Minister Sheikh Hasina.

The BNP boycotted the general election in 2014 that saw Hasina return to power, but Zia is expected to contest the upcoming poll in December.

Her son Tarique Rahman, who lives in exile in London, has been named as the acting opposition chief, despite being convicted and jailed in absentia for ten years on the same charges as Zia.

Last month prosecutors sought the death penalty over his alleged role in a deadly 2004 grenade attack that injured Hasina.

Zia and her son were detained by an army-backed government in 2007 and spent a year and a half in detention pending trials for alleged corruption.