The leader of the Bangladesh Nationalist Party, Khaleda Zia, has demanded the immediate release of her arch political rival, Sheikh Hasina.
Ms Zia said that the circumstances of Sheikh Hasina's arrest earlier this week were "deeply disheartening".
Correspondents say that Ms Zia's comments are highly unusual given the high level of animosity between the two former prime ministers.
Sheikh Hasina faces corruption charges from her time as PM from 1996 to 2001.
She is also an accused in the killings of four political rivals. She strenuously denies all the allegations made against her.
'Indecent situation'
In a statement released on Wednesday night reported by the English-language Daily Star newspaper, Khaleda Zia said that she regretted the failure of the government to protect Sheikh Hasina's "dignity" when she was arrested and taken to court on Monday.
"I am deeply disheartened to see that being a former prime minister, chief of a political party, daughter of a national leader, an aged woman as well as a distinguished citizen of the country, she [Hasina] faced a disgraceful and indecent situation on the court premises," she said.
"It hurt all conscious people and destroyed the image of the government both at home and abroad," she is reported to have said.
"The situation could have been avoided if the government had dealt with the matter more carefully."
Bangladesh crisis Q&A
The BNP leader - who for much of the past decade has been bitterly at loggerheads with her Awami League rival - said that there had been no need to arrest her and send her to jail.
"I was hurt by the illogical, non-political and indecent speeches that Sheikh Hasina made against me and my family many times, but I am also hurt in the same way to see her facing this kind of unexpected behaviour," she said.
She called on the military-backed caretaker government to re-introduce democracy as soon as possible, and urged it not to extend the "unexpected" state of emergency imposed in January.
Khaleda Zia also faces criminal charges over tax evasion allegations.
The government says it wants to rid Bangladesh of corruption before elections are held by the end of 2008.
More than 150 prominent politicians, civil servants and businessmen have so far been arrested.
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