Thousands of opposition supporters staged protests across Bangladesh on Monday demanding immediate release of their leader and three-time former premier Khaleda Zia, jailed early this year for graft.

Zia, 73, is currently on trial on more corruption charges in a makeshift courtroom inside a 19th-century British-built prison where she is the only inmate and in failing health, her lawyers say.

A police official told AFP that some 4,000 activists of the main opposition Bangladesh Nationalist Party (BNP) joined protests outside the National Press Club at the heart of capital Dhaka.

Thousands more joined similar demonstrations in cities and towns across the country.

But opposition spokesman Fakhrul Islam Alamgir has said some 20,000 protesters, many shouting slogans calling Zia's jailing illegal and a sham, turned up at the Dhaka rally.

“We have said the charges against her were false. She should be released immediately and be treated at a private hospital,” Alamgir, secretary general of the BNP, told AFP.

Zia, already serving a five-year term, attended a hearing last week in a wheelchair, telling the court she was extremely ill and is losing feeling in her hand and in a leg.

Her lawyers have said that trying her in the abandoned prison is unlawful.

They met with the chief justice on Sunday to lodge a complaint.

Zia was granted bail in a corruption case in May but remains in jail while she fights dozens of other violence and graft charges.

The Dhaka Central Jail in the old Mughal district of Dhaka was built under British colonial rule in the 19th century and was Bangladesh's largest until it was declared abandoned in 2016.

Zia was jailed for five years in February after being convicted of corruption, a sentence that triggered clashes between police and thousands of BNP supporters.

She was found guilty then of embezzling money intended for an orphanage, a charge she dismissed as politically motivated.

Zia is appealing against the verdict which bars her from standing in a general election to be held in December. Alamgir has accused the government of launching a crackdown on the party ahead of the elections.

“Some 1,200 of our activists have already been arrested,” he said, adding the detentions were aimed at manipulating the polls.