The BNP chairperson, currently visiting the UK, made the call in a Twitter post on Wednesday.
The death toll from floods in northern Bangladesh rose to 107, the government said.
The number of people affected by the floods in 22 districts crossed 3.3 million, according to the latest data from the Department of Disaster Management.
She said flood victims must not be left to "the mercy of a corrupt government".
"I ask everyone especially BNP leaders & activists to urgently help them."
The BNP chief, who underwent an eye surgery at London's Moorfields Eye Hospital last week, was staying with her eldest son Tarique Rahman.